Notes & Comments Glues

From "Construction Notes" Falco Builders Letter, March 1996

Neil Kowald reports, "I am now using a resorcinol called Resobond, however as with any resorcinol, it has no gap-filling ability, requires humidity in the 35-60% range and temperatures above 20°C (and preferably 30°C). It also must have clamping pressure. When these criteria are met (which is not difficult given our climate here in South Australia), the joints are superb.

"You may be interested to note that I read that you must not permit the mixed resorcinol to come into contact with copper! You might like to put a note in the newsletter to advise other builders, as I don't think this is commonly known.

"I have done a lot of research into adhesives due to my original supplier quitting their CAA-approved resorcinol product (as you know Aerolite is banned in Australia), and it seems that many builders and LAME's regard Ciba Geigy K134 epoxy as giving excellent results. K134 is also a CAA-approved glue here in Australia. In cases where it is difficult to attain the required pressures (for example, the trailing edges of the flaps and ailerons), I will be using K134 adhesive. This advice came from Wayne Milburn who assisted Guido Zuccoli."

From "Construction Notes" Falco Builders Letter, September 1996

There's a bit of a controversy swirling around Aerolite glue these days, thanks to a couple of articles on glues. In the June 1996 issue of the EAA's Experimenter magazine, there's an aricle by Bob Whittier entitled "Exploring the Wide World of Adhesives". The article is generally an excellent summary of the history of adhesives.

In pursuing the subject of urea-formaldehyde glues (Weldwood Plastic Resin and Aerolite are the two most popular brands), the author made contact with Victor Boyce, an Australian now living in Florida. If you have made the rounds in the woodworking shops of Oshkosh or Sun'n'Fun, you've undoubtedly run into Victor, who has waged a one-man campaign against Aerolite, and he was largely responsible for the banning of Aerolite in Australia.

I've listened to Victor, and I've read the material he has sent on the investigation of Aerolite in Australia. The evidence is quite clear that there is some weakening of the glue, over time, in very hot and humid climates. At about 120°F, urea-formaldehyde glues begin an oxidation process which makes it become weak, and it tends to decompose into something that resembles brown sugar. The problems occurred with dark-painted airplanes that were left out in the sun for long periods of time, while airplanes painted with lighter colors or which were hangared have not suffered.

In his article, Whittier says "...the FAA now considers UF glues to be unacceptable" and later "and that is why Aerolite glue was banned some time ago in Britain and related countries, and why the FAA now takes a dim view of Weldwood, Craftsman Plastic Resin and other UF glues." And in a recent article in Sport Aviation, Tony Bingelis said "now it seems that the FAA will agree with Australia that the glue should not be used in homebuilt aircraft."

I asked Ben Owen, the EAA's director of information services, about the 'FAA ban on Aerolite and urea-formaldehyde glues'. Ben said that to his knowledge it was not true. We called Tony, who said he was "deliberately vague" in the article and said his source was the article in Experimenter. (Tony also said he was through with writing articles for Sport Aviation, and that he had written his letter of resignation. Ben and I both laughed-we've all heard this many times before from Tony!)

So from what I know, this 'FAA ban' is simply not true.

It's also not true that the glues have been banned in England. I asked Francis Donaldson, chief engineer of the Popular Flying Association about this, and he replied: "No, we certainly have not banned Aerolite in the UK and, in fact, Aerolite and Aerodux are the standard approved glues for construction and repair of wooden aircraft and gliders. Apart from these two, we are now accepting various epoxies (T88 and West Systems) as these have crept into use via various kitplanes, such as the Minimax, Loehle and Fisher kits rather rely on the gap-filling and fillet properties of epoxy."

"Our view is that Aerolite is perfectly satisfactory for the lowest ambient temperatures found in the UK, and is particularly good with the damp conditions for which we are well known! We have not come across any cases of glue failure due to excess temperature."

The wing ribs of the Pitts Special are today built with Weldwood Plastic Resin glue, they always have been, and it's a fully FAA certificated design. Stearman wing ribs have been made for 50 years with the stuff. Production Falcos from the 1950's and 60's were built with Aerolite, and there's been no pattern of glue problems.

I think the Australian authorities were correct in their observation that urea-formaldehyde glues weaken when exposed to long periods of high temperature and humidity. However, I think the ban on Aerolite was ill-considered. They banned acid-catalyzed urea-formaldehyde glues while permitting other urea-formaldehyde glues. Yet the acid is a catalyst that disappears in a few days and leaves only cured urea-formaldehyde. The Australians acknowledge this disparity, yet they ban the glue on the basis that 'other acceptable glues are available'.

Therein lies the rub. There's one thing that all knowledgeable people agree on-the more you know about wood glues, the more you realize that there is no one perfect glue. All available glues have their strengths and weaknesses. Epoxies are thermoplastic. Weldwood Plastic Resin glue is cheap, easy to use, yet adheres poorly to birch plywood. Only resorcinol is truly waterproof and unaffected by heat, but it's difficult to use in small quantities, requires higher curing temperatures and has very poor gap-filling characteristics.

At the end of the day, our advice is still the same: learn the strengths and weaknesses of the available glues and pick the one that's best for you. We use Penacolite resorcinol for all spruce laminations, and Aerolite for everything else. Lots of people use epoxies in their Falcos. There's no doubt that the glues are user-friendly and excellent adhesives. The principal concern with the epoxies is that they soften with heat. If your plane is painted a light color, there's no reason for alarm, but if you paint an epoxy-glued Falco a dark color, you're taking a needless risk.

From "Mailbox" Falco Builders Letter, December 1996

You haven't lost your touch for stirring me up. Your comments in your Construction Notes said that I was "deliberately vague" in my article (true). You also added: "and said his source was the article in Experimenter". Not true. I only mentioned Bob's article to you because I thought it was worth reading, and I didn't know if you had seen it.

Anyhow, I view this as another instant in our long association where you typically leave the impression that Tony's comments/suggestions are shallow assumptions not worthy of note. Actually, my "vague" treatment and conclusion on the urea-formaldehyde subject was based on the following:

1. The draft revision of AC43-13-1B (April 4, 1996) in which I participated. However, I did not comment on the FAA's paragraph which I quote in its entirety: "Plastic Resin Adhesive. Although 'plastic resin glue' (urea-formaldehyde resin glue) has been used in wood aircraft for many years, caution should be used due to possible rapid deterioration (more rapidly than wood) in hot moist environments or under cyclic swell-shrink stress. For these reasons, urea-formaldehyde should be considered obsolete for all repairs. Any proposed use of this type adhesive should be discussed with the appropriate FAA office prior to using on certificated aircraft."

2. Information I had previously seen in the Australian magazine Air Sport.

3. Information received from Victor Boyce (Corby Starlet Newsletter Technical Editor) back in 1988. He was the "expert witness" for the government. This eventually led to the Department of Aviation banning the use of this type of glue for aircraft construction.

4. Australian Airworthiness Advisory Circular, No. 108, January 1979 regarding the Australian prohibition of ACP or UF glues.

And for the last laugh, I am now preparing the December issue article for Sport Aviation. It is the last one. EAA believes it. They invited me to a farewell banquet November 1.

Alfred, stay in touch. I'll need the stimulus.

Tony Bingelis
Austin
Texas

I didn't mean to misquote Tony, irritate him or demean his views in any way. Ben and I were merely curious about the validity of the "FAA's ban on Aerolite". I did not know of the proposed wording in the AC43-13, and I wish Tony would have mentioned it because I certainly would have used it in the article. I didn't mention the other sources which discuss the merits of Aerolite, because I was mainly interested in whether the FAA has banned Aerolite, which is what builders were perceiving from the various articles. The AC43-13 document applies to certificated aircraft, and thus there is no 'ban' on the use of Aerolite in homebuilt aircraft.

Tony can have all the farewell banquets he wants, but I ain't going to believe it till I see a year pass without more articles!

Alfred Scott

From "Construction Notes" Falco Builders Letter, December 1996

More on the Aerolite 'controversy'. Much of the misunderstanding about Aerolite rose out of two articles, one by Bob Whittier in the EAA Experimenter and another by Tony Bingelis in Sport Aviation. Both articles had a lot of merit, but both left the impression that somehow the FAA was banning Aerolite.

I was not aware that a proposed copy of the FAA's AC43-13 was being passed around for comment. This document covers acceptable methods of repair for certificated aircraft, and in the proposed document, the following language is included: "Plastic Resin Adhesive. Although 'plastic resin glue' (urea-formaldehyde resin glue) has been used in wood aircraft for many years, caution should be used due to possible rapid deterioration (more rapidly than wood) in hot moist environments or under cyclic swell-shrink stress. For these reasons, urea-formaldehyde should be considered obsolete for all repairs. Any proposed use of this type adhesive should be discussed with the appropriate FAA office prior to using on certificated aircraft."

Let's remember that this document governs the methods used for repairing certificated aircraft, and while it is a good guide for all types of aircraft, it is not binding on kit or homebuilt aircraft. I've had some correspondence with Bob Whittier, and he has written an excellent article on Aerolite that is in the December 1996 EAA Experimenter.

My view of Aerolite remains the same. It is good that we are informed about the limitations of the glue, but I think that when you consider the limitations of all of the glues available, Aerolite should still be considered for use. Any blank prohibition of Aerolite would be like banning people with bad tempers or body odor. That would deprive the world of people like Bill Gates (or some might suggest, even me!).

Bob Whittier's latest article included some new and interesting information on West System epoxy. When I last spoke to the folks at Gougeon Brothers, they were not able to supply specific information about the temperature performance of their West System epoxy. They generally felt that it has slightly superior temperature performance when compared to other epoxies like T-88, but certainly not greatly different.

Thomas Pawlak of Gougeon's Technical Services reports: "We believe our epoxy is suitable for use in wooden aircraft if you are aware of its physical characteristics and take precautions to deal with its limitations. It has been used successfully for 20 years to build experimental aircraft. We broadly recommend our epoxies for many critical high-strength applications such as aircraft construction and repair. We also broadly caution all epoxy users to optimize contruction methods and details to get the best results."

"All room-temperature-cured epoxies will soften when exposed to elevated temperatures. Most composite materials soften to varying degrees when exposed to high temperatures. That is why many composite structures including aircraft are painted white or near-white, to reflect most of the sunlight's energy rather than absorb it into the substrate. Even so, surface temperatures can exceed the heat deflection temperature of the epoxy. Fortunately, as soon as it cools to room temperature the epoxy returns to full strength."

"Our West System epoxy has a heat deflection temperature (HDT) that ranges from 118 to 123°F, depending on which hardener is used. This temperature is determined by means of a simple test. A bar of neat epoxy (epoxy without thickeners or fibers) is supported at both ends on triangular blocks and a load is applied to the midpoint. This setup is immersed in oil that is gradually heated until the specimen being tested bends 0.10 inch. The temperature is recorded and is that epoxy's HDT"

"Testing done by us shows that West System epoxy returns to full strength after cooling to room temperature as long as the temperature of the epoxy has not exceeded 200°F for long periods and as long as a joint it is attached to is not subjected to durational load during the heat cycle."

From "Mailbox" Falco Builders Letter, December 1996

You may have got the impression that I use a lot of epoxy; you are right. The epoxy I use, Araldite AW 134, is produced by Ciba-Geigy in Australia. It maintains its strength up to 80°C and is still at half strength at 100°C. Given that the glue is considerably stronger than necessary to begin with there appears to be little risk of glue failure due to temperatures even in our climate. Add to that the margin of strength in the aircraft structure, and the rapidity of cooling in flight, the risk at tached seems slight. There are two rules I intend to follow however. Never take off with a jet of steam coming from any breather hole, and no more than 3 Gs immediately after takeoff. If the thing falls to pieces before leaving ground, injuries should not be serious.

There are many joints in the Falco that I-and I am not very skillful I admit-find very difficult to make really accurately and which are also difficult to clamp with adequate pressure. With my level of skill, I believe that had I used resorcinol throughout there would have been many more poor joints in the machine than I have made with epoxy and at 80°, or even 100°C, the machine will be safer than it would have been with resorcinol.

Ian Ferguson

Ian also sent along product literature on Araldite AW 134. The literature is impressive and gives charts (see above) showing the strength of the glue vs temperature-that's something I've never seen before in product literature on epoxies.

Scoti

From "Mailbox" Falco Builders Letter, December 1996

A tool I have found to be very useful is a balance beam for mixing glues and resins of various types. [Ian's balance beam is a board pivoting on a fulcrum, but with a moveable balance weight on one end that's mounted on a threaded rod.] The procedure is to balance two cups, the one to contain resin on the mark 100mm left of the fulcrum, and the one for the hardener the appropriate distance to the right-500mm in the case of a 5-1 ratio. These are balanced empty using the moveable counterbalance to the left of the resin site. The required amount of resin is placed in the left cup, and sufficient hardener to balance in the right cup. The right end of the beam can be marked for any ratio needed.

A problem associated with liquid hardeners is the error associated with the residue left in the resin container, especially in small batches. This can be overcome, or eased in two ways. The first is to mix in both cups, i.e. tip the hardener into the resin and mix, then tip the mixture back into the hardener cup and mix again. Not 100% and wastes a lot of glue, especially in small batches where the error is most important.

Another solution is more elegant but somewhat involved. You will require a 100mm mark to the right of the fulcrum on your beam, and mixing cups all of which weigh the same. The resin is placed as before to the left of the fulcrum, but this time water is added to the hardener cup to balance. The resin and the hardener cup are removed and the beam is re-balanced with two empty cups each at the 100mm mark. The empty cup is then removed from the left side and replaced with the cup of resin. Water is then added to the right cup to balance the water in the right cup, and then add hardener to balance to the left (resin) cup. A bit more tedious no doubt, but think of the money saved in disposable cups. You will destroy only half the number!

Ian Ferguson

In the U.S., most people who use epoxies have metering pumps that dispense the proper amount of resin and hardener, so Ian's elaborate procedure is something you don't have to worry about. You just put the mixing cup under the pump and pump away.

Scoti

From "Construction Notes" Falco Builders Letter, June 1998

Tom Webber asks if the 4:1 ratio of Aerolite powder to water is critical, and he wants to know what happens if it is too thin or wet, and whether this will compromise the strength of the joint. Our experience is that it is not critical. It's best to start with a 4:1 ratio and to know from that experience what a normal consistency is for Aerolite.

As you store Aerolite, it will become progressively thicker. It's okay to add a little water to keep the consistency the same, and as long as the mixture will flow, it's still an acceptable glue. If you're going to use the glue in an application where you want to spread a large quantity with a brush, then it's fine to add some water to get a brushable consistency.

From "Mailbox" Falco Builders Letter, June 1998

During the past couple of weeks, I've been trying to locate the Penacolite G-1131 resorcinol/formaldehyde adhesive to use on my Falco. As shown in the price list, this is supplied by IndSpec Chemical Company, and prior to that by Koppers. I wanted to let you know that it has changed hands again. Borden Chemical Co. has purchased the rights to G-1131 and is now the sole manufacturer.

Unlike IndSpec, Borden does use distributors for their products and after contacting them directly (800-346-2546), I was put in touch with their west coast distributor, Din Tech. The sales point of contact is the owner (Denny), who can be reached at (562) 908-5554, ext. 215. Initially, Denny was unfamiliar with the resorcinol name and identifier (Borden has several other adhesives and all have product numbers like RS240MD). It took over two weeks and the help of a chemist at Borden to determine that Penacolite was available and that the product identifiers have not changed (G-1131A & B).

I would suspect that other builders looking for Penacolite will find similar difficulties if they have to go through distributors other than Din Tech. Until each is made aware of the new product, there will be some delays. The good news is that the pricing hasn't changed for the resin and hardener, and I believe that credit card purchases will be accepted. While I may not have given you all the infomation you'll need to update the next price list, I hope that my experience will help someone else out a bit.

Lance Roundy

From "Construction Notes" Falco Builders Letter, March 1994

(Note to myself, I have a couple of photos of this glue.)

There's an interesting new adhesive on the market, a product known as Bison Timber-Tix. I first heard of this glue from Jean Peters at Western Aircraft, who raved about the glue.

Timber-Tix is a one-component glue that comes in a 50 ml tube or 310 ml cartridge (for use in a standard caulking gun). A clear, thixotropic material of toothpaste consistency, Timber-Tix is a moisture-cure polyurethane adhesive that hardens from the moisture in the air or in the wood. It may be used to glue a wide variety of materials: wood, concrete, expanded polystyrene, glasswood, formica and metals on porous substrates.

Timber-Tix is advertised as weather- and seawaterproof, and meets Din 68602: B4 standards (whatever they are). The glue is gap-filling and said to be extremely strong. To use, you apply a bead of glue with the caulking gun, troweling the glue around as necessary. You must assemble the parts within 30 minutes and the parts should be prevented from moving by clamping slightly. Handling time is 2 hours at 20°C and setting time is 24 hours at 20°C. The lower the temperature, the longer the setting time. Mininum application temperature is 5°C.

This adhesive has become very popular for building wooden boats, where it is used in place of epoxy. It is manufactured in Holland by Perfecta Chemie B.V, P.O. Box 160, 4460 Ad Goes, Holland. Tel: 01100-31944, Fax: 01100-32077. The U.K. distributor is Bison Adhesives U.K., Rowberry House, Copse Cross Street, Ross-on-Wye, HR9 5PD, England. The North American distributor is Trans World Distribution (attn: Charles Sinclair), 424 35th Avenue NE, Calgary, Alberta T2E 2K7, Canada. Tel: 403-277-2554, Fax: 403-277-2544. Western Aircraft sells the adhesive and is probably your best contact for small orders.

Whenever a new product comes on the market, there's a tendency in people to rush to embrace the 'new boy in town' as if it will cure all problems. My experience with adhesives, however, is that there are no easy answers and that simplistic answers are always wrong. Each glue has its own characteristics, strengths and peculiarities, and it's important to get to know a glue before you start using it for important applications.

We don't know, for example, if this glue is thermo-plastic like epoxy, but apparently it is not. The manufacture claims temperature resistance up to 125°C, and if that's accurate, then the glue has ample temperature resistance for our use. We don't know how it behaves under heavy clamping pressures. We don't know if the strength is affected by long-term exposure to high temperatures.

Based on what I know so far, however, this glue is well worth investigating, and I suspect that it will eventually turn out to have a number of applications where it would be an excellent choice.

From "Mailbox" Falco Builders Letter, June 1994

After reading the Falco Builders Letter, March issue, I feel I have to express some of my experience with Jean Peter's epoxy system. The comments by George Richards and Alfred Scott leaves a big grin on my face because of someone not really doing any hands-on research and becoming an armchair critic.

Over the last 10-12 years, I have had considerable experience in polyester, vinyl ester, Safe-T-Poxy and the West System. All are good systems under controlled conditions and personal protection as to odor and toxicity. Some of these systems are very allergenic. I have used the Peters system for eight years. During this time, it has been used to build a KR2. Many, many test pieces have been fabricated for my own testing and for our Dept. of Transport. My propellers have all been glued with this system. The prop that is presently on the KR2 has over 100 hours on it without any sign of failure or delamination.

With cowlings made with this resin, I had to prove that it would not sustain a flame. Our tests for the DOT were run with an acetylene torch; it burned the resin but would not sustain a flame. The cowlings after a few flights reached a full cure because of the engine heat, and became firm as a cowling should.

With the use of an autoclave, two booms have been constructed for an Aero Magnetometer company. One five feet long and the other 14 feet long, using the Peters system. The five-footer is mounted on the fin of a Cessna 172XP and has been in use for 3-1/2 years; the 14-footer had to be shortened to 12 feet to get it in the proper position on the resonance curve and has been in service for two years mounted on the fin of a Cessna 310. On the last inspection, no fatigue or delamination was found on either boom.

This system mixed with microballoons has been used as a light automotive body filler with excellent results. Another good test was made when a friend, with a leaky exhaust muffler on his motorcycle, wrapped the muffler with fiberglass and the Peters system. After three years, when the bike was sold, there was no evidence of any failure of the muffler.

I would challenge the both of you and the manager of Ciba Geigy (who probably does not have any hands-on experience with Araldite 509) to try any of my test pieces and propellers I have constructed for failure of a glue joint. Try it-boil some test pieces in water for 4-5 hours and see if a glue joint fails.

In my opinion, there are two types of people in this world-one who educates himself and experiments, and the armchair types who have not experimented and are an authority with unfounded opinions. This is an unsolicited letter, but I am highly biased to this proven system with justification.

Adrian H. Carter
Calgary, Alberta
Canada

We're delighted to publish this glowing report, however you're offering anecdotal evidence. I'm still interested in seeing independent test data. I'm sorry, but I'm too busy to entertain the idea of testing adhesives when I can purchase other adhesives for which ample test data is available.

Alfred Scott

From "Construction Notes" Falco Builders Letter, September 1993

Martin Steinwender of Germany asks about using Ciba Araldite epoxy and whether it would be good for wood protection. Also, he had read about not closing all wooden surfaces completely otherwise it cannot breath.

I am not familiar with the Ciba Araldite epoxy, but I will simply mention that some epoxies are very thin and are intended for moisture-protection coatings (West System epoxies are of this type) and other epoxies are thicker and are intended for gluing. Epoxies are excellent for moisture protection, however if the resins are thick, they can add a lot of weight to the airplane, and that is obviously something that you do not want to do.

On the question about not closing the wood surfaces so that it cannot breath, this is something you need to use your judgement on, and it will depend on the volume of the air space within and the area of the plywood. Large areas should have small vent holes so that air can escape, while smaller areas don't need to be vented because the volume within is so small.

Keep in mind that all wood is porous and even when you have a solid piece of spruce, you have air trapped in the spruce. It does not have to all get out.

From "Mailbox" Falco Builders Letter, December 1993

Regarding the question from Martin Steinwender of Germany about Ciba Geigy Araldite. This epoxy is excellent and is commonly used in South Africa. Of course, the specific system is important-AW106 and 953U are the two relevant components in a 50/50 ratio by volume. Neither I nor Fanie Hendriks have ever had a glue failure, even on oak/walnut test blocks of shear and peel types.

Before starting the Falco, I had structurally completed a KR2 which was subsequently completed and flown about four years. Two years ago, the owner suffered carburetor ice, which resulted in a forced landing, wing tip strike, cartwheel, and totalled airframe. The wreck was rigorously examined and not one glue failure existed.

Use heaters if necessary to keep the curing temperature above 20°C. Glue lines must, of course, be thin, say 0.3mm max.

Brian Nelson
Randburg, South Africa

From "Construction Notes" Falco Builders Letter, September 1992

Stuart Gane was alarmed to discover a warning that West System 410 Microlight filler is sensitive to elevated temperatures and that the material can swell when hot. Stuart writes, "I have been informed by Brian Knight of Gougeon Bros that the filler is probably going to be OK in the temperate climate of the UK, if I were to paint my Falco red. However, he did warn me that if I were to fly my aircraft to, say, the Mediterranean or wherever the air temperature exceeded the 70-80°F range, I could expect the sanding marks left by the rubbing-down process to show through the paint or, even worse-where there is a substantial thickness of Microlight filler-a swelling of the filler could occur, because the filler is a thermoplastic material. Only light colors would overcome this problem." Glass and phenolic microballoons are not affected in this way.

From "Construction Notes" Falco Builders Letter, March 1990

Bob Bready sends word that you can have a problem using a "salamander" heater in your shop with epoxy. He varnished the surface of the wing with West System epoxy and noticed that when it hardened, the surface was gummy. He called Gougeon Brothers, and they attributed the problem to the heater, which they say will put moisture into the air.

Buzz Glade had the same problem in humid Florida and cured the problem by using a heat gun. As he squeegeed the epoxy into the fabric, he used a heat gun in the other hand. The heat thins the epoxy so that it is much easier to spread, and the heat tends to kick off the chemical reaction so that the epoxy cures to a hard sheen.

From "Construction Notes" Falco Builders Letter, September 1998

Ben Owen at the EAA reports that he recently purchased some Penacolite glue from Borden, the new manufacturer of our favorite resorcinol glue. In the process, he had to chase down a retailer of the glue, and he passes on the retailer's name for Falco builders: Jeff Pitcher, Custom Pack Adhesives, 11047 Lambs Lane, Newark, OH 43055. Telephone: 1-800-454-5583, fax (740) 763-2888.

From "Construction Notes" Falco Builders Letter, September 1991

I guess everyone who works with epoxy has discovered PR88 hand creme. It's great stuff, you rub it all over your hands and it dries to a barely noticeable film. You can work with epoxy or paint and then just wash it all off with water.

PR88 is made in Germany, and a company in the U.S. has cloned it. The new stuff is called Series 8 cream. I bought some because Wicks and Aircraft Spruce no longer carry PR88, and as far as I can tell, it's the same stuff except for a slight change in the fragrance.

From "Construction Notes" Falco Builders Letter, December 1991

The latest issue of Highland Hardware's excellent catalogue talks about the new Titebond II waterproof glue. Titebond was the first of the yellow glues, known as aliphatic resin glues, and sold as 'Carpenter's glues' by some brands. They're superb for general woodworking around the shop because they're one-part glues that you just spread and clamp. They are stronger than the white polyvinyl glues like the original Elmer's and also sand nicely, while the white glues gum up your sandpaper.

The problem with all of these general-purpose glues is that they have minimal resistance to water and heat. Although they are calling it 'waterproof', apparently Titebond II is a water-resistant glue that passes Type II (above waterline) waterproof testing. That puts it in the same category with Aerolite and plastic resin glue (only resorcinol and high-temperature phenolics are waterproof). Also I don't know a thing about claims for temperature performance of Titebond II.

At this point, no one should use Titebond II on their Falco, but I do think it's a good idea to get some, use it for general projects and run some tests on the glue for water-resistance and temperature performance. Lord knows, there's no easier glues to use that Elmer's or Titebond, and if there's the slightest chance they're suitable for use on airplanes, we should all take a look-see.

From "Construction Notes" Falco Builders Letter, December 1990

Gary Wilburn asks if anyone has tried using Hydrocote Exterior Polyshield varnish on their Falco. It's sold by Highland Hardware who sell it for general woodworking applications. The Hydrocote finishes are unusual in that they are water-based polyurethanes. They have an interior polyurethane and the exterior-rated Polyshield.

Gary spoke to Highland Hardware and asked about the finishes. They said the varnish has water-exclusion qualities that are equal to or better than conventional polyurethanes but didn't know enough about the application on aircraft to make a recommendation.

I'm always suspicious of new miracle finishes that are not used by professionals. Lacquer-type finishes harden by vaporization of the solvents. Enamels harden by a molecular change in the resins in the paint that cause them to become a solid material. Epoxies and polyurethanes work by the chemical reaction of two completely different resins which react chemically to each other and which then form a complex solid molecular structure by cross-linking of the molecules. The entire film surface consists of interwoven spiderwebs of cross-linked molecules.

So whenever I hear about one-part epoxy or polyurethane finishes, I wonder if I'm being sold a bill of goods. If there are two highly reactive resins in the paint, how do they know when to 'get married'? I've heard of such paints as moisture-cure polyurethanes that don't react until moisture in the air gets to them, and when someone explains to me that that is the way the finish works, I can understand it.

But when I buy a can of Minwax polyurethane varnish or shake-and-spray cans of epoxy paint, I have this strong suspicion that all I'm getting is a normal paint with some ground-up polyurethane or epoxy powder sifted into the paint. I can't believe that there is any cross-linking taking place at the time the finish is hardening or that I'll get the extremely durable film that these finishes are famous for.

Until someone explains to you that cross-linking takes place when you spread the stuff, I think you're safe assuming you have polyurethane-powder-filled paint in the can and that the polyurethane did its crosslinking at the paint factory.

From looking at the sales literature, the primary advantages that I can see about the Hydrocote finish is that it is very thin, easily applied, easy to use and cleans up easily with water. Until I saw some data on the finish, I would assume that it has no special moisture-exclusion capabilities over other similar one-part polyurethanes.

There's a standard moisture-exclusion test developed by the Forest Products laboratory (described at the bottom of page 9-2 of our construction manual) that's not the easiest thing for one of you do but if the paint manufacturer is making moisture-exclusion claims, they should be able to supply test data-if they can't they're talking through their hat.

A quick-and-dirty test that you can do that'll tell you something is to coat one side of 12"-square pieces of thin birch plywood with a number of finishes-West System, spar varnish, latex paint, Minwax polyurethane, and your new 'experimental' finish. Lay these on a carpeted floor, finish side up, and put moist paper towels on top of each piece. Observe which of the pieces of plywood curls up and which lays flat.

It's not much of a test but it will show you the relative values of each type of finish, and you'll quickly be able to sort out the worst and best.

One thing I've always liked about West System is that the Gougeon Brothers company has a testing laboratory, publishes test data and explains how their finishes work. Everyone knows that there are only three manufacturers worldwide for the base epoxy resins (Dow, Shell and Reichold, if memory serves) so everyone buys from them and mixes their own brew. There's considerable technology in the recipe and anyone who can't back up claims with data should be considered to have a certain proportion of snake-oil in the recipe.

Gougeon Brothers explains that their secret is in the reactive diluents-a strange but correct word that simply means that the thinners don't evaporate. Thinners that evaporate leave a porous film with tiny pinholes, while the West System's thinners 'get married' with the resins. This is almost exclusively the reason why West System epoxies are so good for moisture protection.

The fall 1990 issue of Gougeon Brothers' Boatbuilder shows the results of some tests that they ran on the moisture exclusion effectiveness of West System 105/205 as compared with other finishes, and remarkably enough they also offer a free booklet, Moisture Exclusion Effectiveness, on how to test coatings for moisture exclusion. Write Gougeon Brothers, Inc., Dept 91, P.O. Box 908, Bay City, MI 48707.

From "Construction Notes" Falco Builders Letter, March 1989

Craig Bransfield and Robert Brantley, each a modeler-turned-Falco-builder, have touted the instant glues made by Pacer Technology. Zap-CA is for any hard non-porous surface, Poly-Zap is for acrylic, polycarbonate and fiberglass, and Zap-A-Gap is the formulation for wood. I have bought them all and tried them in our shop, and while I like the glues, I have had mixed results.

I use a lot of acrylic to make router templates for ribs, and frequently I have to build up several layers of the stuff for cross-brace and gusset guides, which have to be precisely located. An instant glue is a boon for this sort of thing. I have had better luck with Zap CA, then 'hottest' of the glues, than I have with Poly-Zap which is intended for flexible materials. I'm sure it's something that I am doing wrong, but I find the glue a little unpredictable. In some cases, the glue hardens in 30 seconds, and in other cases it takes 5 minutes-I suppose the surface is contaminated with oil or something.

There is also a noticeable characteristic that the glue does not form a homogeneous film bonding the two pieces, but rather takes on a lacey pattern of interconnected islands. In many cases, I will go back later and work in more glue by capillary action to get more coverage.

I use a lot of Zap-A-Gap to glue pieces of wood in my cutting fixtures. In these situations, I want to place a stop block, glue that sucker down and start using the tooling right now. Zap-A-Gap is nice because it is so thin that you don't normally have to contend with a hardening blob of glue squeeze-out. Glues such as Elmer's white glue or the yellow aliphatic resin glues harden from the outside, while cyanoacrylates like Zap-A-Gap harden only when deprived of air. Any extra glue is easily wiped off with a paper towel.

I squirt a little of the glue, position the block and then grab my Senco stapler and shoot a staple to hold it in place until the glue dries. The label on the bottle says that the glue will harden almost instantly, but I haven't found this to be true. The only thing this glue bonds instantly to is your fingers. For wood, I think at least 10 minutes is necessary. That's too long for me since I want to use the fixture right away, so I just depend on the staple and go right ahead and use the tooling to cut the pieces. The whole process only takes 15 minutes, so I suppose the glue will help on the next fifty Falcos.

I also think the strength of these glues is exaggerated. We use a lot of Baltic birch plywood for our tooling, and we glue the pieces together with a variety of glues. On our router jigs for the aileron/flap leading edge ribs, we needed a block of wood stuck on one side as a safety feature to preclude the possibility of slipping and routing off your right thumb. We installed half of the safety blocks with Aerolite and half with Zap-A-Gap. All of the Zap-A-Gap blocks broke off in service, while none of the Aerolite blocks did.

When you look at the failures, it is apparent that the same lacey pattern of hardened glue is one of the culprits. And in chipping away hardened pieces, I have come to the conclusion that these glues are medium-strength glues, roughly in the same category as Duco Cement or Elmer's white glue.

We have gradually gone to Aerolite for all of our gluing of the jigs and fixtures, except for the staple-glued guide and stop blocks of our cutting fixtures. One thing that I like about Aerolite is the way you can clean up the squeeze-out at various times. The white and yellow glues skim over and are difficult to clean up, and I prefer to scrape and then wipe with a wet paper towel. When the glue is freshly applied, you can wipe Aerolite with a wet paper towel, or you can wait until the glue is hardened to a gummy, cheese-like state and then scrape it off.

I have also given up on using the 'instant' glues on my acrylic pieces. I find that the old standby of methylene chloride, which has been used for years for bonding acrylic, works better. I like it because you position your piece of acrylic and then run a bead of the thin liquid around the edge. It flows in by capillary action. You get good coverage and in 30 seconds it is hardened enough so that you can let go of things.

Stephen Friend writes from Australia that acid-catalyzed phenolics (ACP) and/or urea formaldehyde glues are banned there for joints in the primary structure of wooden aircraft. The prohibition is based on some problems with ACP, a glue type about which I know nothing. The theory on that glue is that the acid slowly attacks and weakens the wood. ACP glues have been implicated in accidents.

The Australian report goes on to say that the case against UF glues is not so clear-cut. They cite no glue failures or accidents and base their prohibition on UF glues because "they are known to react chemically to changes in humidity and there is ample research to cast doubt on their long-term durability in outdoor conditions." While they recognize that some formulations are more durable than others "the Department has not yet seen data to prove that any particular formulation does not suffer long term deterioration, particulary under outdoor conditions."

The Australian position of Aerolite and other UF glues is truly absurd. I don't know a thing about the phenolic glues, but I suspect that the acid is much stronger than the very weak acid used with Aerolite. Several years ago, there was some concern about mixing epoxy and Aerolite since epoxies such as T-88 are slightly alkaline and their cure is completely prevented by any acid condition. If you put epoxy over a fresh Aerolite joint, the glue would not harden, but Falco builder and chemical engineer John Oliver ran a series of extensive tests and found that this effect stopped after 48 hours. In short, there is a little acid left in the wood after the glue is hardened, but it quickly disappears. I am not aware of any evidence that the acid-weakening of wood from Aerolite's catalyst is a problem for anyone but nervous bureaucrats.

The statement that UF glues will not stand up under 'outdoor' conditions is absolutely correct. No room-temperature glue, except resorcinol, will stand up to long-term repeated water soakings on unprotected wood. Aerolite is a water-resistant glue, not a water-proof glue, but this makes no appreciable difference since the wood is so well protected from moisture that it would be a very rare event that a glue joint in a Falco would ever see any liquid moisture.

One thing that Steve Friend can do is to use epoxies, paint his airplane white and take tranquilizers when the temperature is high. I will observe that the addition of some brown food coloring would make the glue indistinguishable from resorcinol. Perhaps they're like my daughter who explained why she so readily dove into a Christmas plum pudding that turned out to be inedible.

Said Sara, "I never question anything that's brown."

Alfred Scott

From "Tool Talk" Falco Builders Letter, September 1989

My initial efforts to use Zap-A-Gap cyanoacrylate glue were unsuccessful, but after I learned how to use the accelerator, I have become addicted to the stuff. It seems absurd to have an accelerator for an "instant" glue, but the simple fact is that Zap-A-Gap takes minutes, not seconds, to harden. The accelerator, called by the silly name of Zip Kicker, comes in a little spritzer bottle. You put the glue where you want it and then fog the slightest amount of accelerator to the other side. Bring the two pieces together and wait about five seconds-that's it. If you want to position things, then apply the glue, bring the two parts together and then fog some accelerator on the glue line. If you have enough glue applied, there will be a little fillet which will harden and then you should leave the assembly alone for some time.

Don't use this glue on your airplane. It's pretty strong, but you don't get solid film at the glue line, and it's relatively easy to break two pieces apart. But for making jigs and temporary holding fixtures when you want to glue a block of wood, metal or acrylic in place now, there is nothing to equal it. If you need an odd-shaped standing stick, you can fog some accelerator on some sandpaper, put the glue on a flat stick of wood, bring the two together, zip around the stick with a knife to trim the paper off flush and start sanding-within 60 seconds or less from start to finish.

From "Construction Notes" Falco Builders Letter, December 1989

I'm hearing nothing but rave reviews on West System Microputty. Everyone who has tried it likes the stuff and has been using it in place of the old faithful microballoons. Microballoons come in two variants, glass and phenolic-the brownish phenolic type are most often used. Joel Shankle says that he thinks microballoons are better for shaping, since they give a good hard shape, but that Microputty is a better filler. It is more powdery, is easier to sand and holds a feather edge better.

When you open a container of microballoons, you'll find a bag of dusty powder which frequently is lumpy, and when you mix it with epoxy there always is a little problem of breaking up the lumps and getting a consistent mixture. Microputty has a slightly creamy feel to it, and it mixes with epoxy very easily. West System Microputty is neat stuff, and I think it's all most of you ever need to use for filling and smoothing.

From "Construction Notes" Falco Builders Letter, December 1989

I've been using some Weldwood plastic resin glue lately for laminating the rudder and elevator tip bows. I mix a fair amount at each time, and it's always best to read the instructions on the container. These instructions, however, are the worst I've ever seen-or at least that's the way it worked out for me. They say to pour a little water into the powder, mix into a thick paste and then slowly add the rest of the water until the glue is the consistency of thick cream. For small batches and a mixing stick, that method works okay, but it's slow, difficult and the mixture is invariably lumpy.

I use a mixing device-a little propeller on the end of a shaft-that I put in the drill press. I've found that if you start with water only, turn the mixer on, and then slowly pour the powder in, it mixes beautifully. You get the nicest, creamiest mixture without a single lump.

From "Construction Notes" Falco Builders Letter, December 1989

When you use resorcinol glues, you really have to measure the two parts by weight and for most people that means making a proportional balance scale. Pictured here is my contribution to the art form, made in one evening of 3/4" baltic birch and glued together with instant cyanoacrylate glue.

This is actually a dual scale which will balance at either 4:1 or 5:1. The powder is put in the cup on the top of the arm and is located 15" from the pivot point. To locate the cup at this point, there's a round disk with concentric circles drawn on the top face.

The liquid is put in a cup below the other end and hangs from one of two points, 3.00" or 3.75" from the pivot point. I have it set up so that it balances with empty cups in place and set up for a 4:1 mixture. For a 5:1 mixture, you move the point from which the cup hands and then put a balancing weight-just a piece of plywood-under the cup.

The pivot point is about 1-1/2" above the pivoting arm and the ultra-low-friction bearing system is nothing more that some quarter-inch aluminum tubing pressed into the plywood and with .125" music wire as the "shafts". Brass tubing would probably have been better, but sometimes you just use what you can lay your hands upon.

From "Construction Notes" Falco Builders Letter, June 1988

If using heat to cure epoxy surface finished in cold weather, be sure to preheat the parts for a day or two prior to spreading the resin. I had some trouble with (apparently) moisture in the wood causing surface bubbles as it tried to escape from the wood under the heat. No problem if the part's temperature/humidity are stabilized at a "curable" temperature and no additional heat is to be applied.

Craig Bransfield

From "Construction Notes" Falco Builders Letter, June 1988

I made up some text glue blocks, but I was unable to break the joint with a vertical sledgehammer blow on the garage floor. I wonder if a simple "tensile strength" test such as that shown on page 60, Illus. 103 of Patrick Spielman's "Gluing & Clamping" would serve the same purpose?

Craig Bransfield

Sorry, Craig, but that test is worthless.

Alfred Scott

From "Construction Notes" Falco Builders Letter, June 1988

Ask the "government" to save you her empty plastic margarine tubs. These are perfect for mixing epoxy (non-stick, and can be re-used), mixing and storing Aerolite resin and hardener, keeping small parts, etc., and the price is right!

Craig Bransfield

From "Construction Notes" Falco Builders Letter, June 1988

Gougeon Bros. disposable plastic gloves are cheap and plentiful, but they withstand almost no mechanical action without tearing out the fingertips. My dentist says that the heavier medical-grade gloves they use are getting very scarce due the the problem with AIDS. I think I'll try to find some dishwashing gloves, which have long cuffs to cover the wrist gap where your shirt sleeves end.

Craig Bransfield

From "Construction Notes" Falco Builders Letter, March 1987

In our last builder letter, John Brooks Devoe asked about the problem of the short shelf life of Penacolite resorcinol. Koppers typically sells the stuff by the tankcar lots. For such customers, they will recertify the glue for an additional year when samples are returned for testing. They test for (a) solvent content in the glue, (b) gel time and (c) shear strength. Since their glue is used for the most critical applications, Koppers takes a very conservative view. If you have kept your cans of glue tightly closed and open them only on occasions, the solvent content is certainly fine for two years. I would suggest you run some block shear tests on hard maple. If the glue joints are good, I would continue to use it.

From "Tool Talk" Falco Builders Letter, June 1987

Does everybody know about glue injectors? I find them quite helpful in one specific situation: when I've bent skin around something like the wing leading edge and didn't get it entirely glued, then need to get glue up into the crack between skin and leading edge in order to reclamp everything and get it fully fastened. (It happens.) Sometimes it's only a pocket a few inches long that needs to be touched up, but it's no problem with a good glue injector. Squirt some Aerolite well up into the void with one injector then some hardener with another.

The only place I've ever seen them is in the Garrett Wade catalogue: they call them "glue syringes" and sell a package of three for a big $3.75 (item #63J01.01). Garrett Wade's telephone order numbers are (800) 221-2942 and (212) 807-1757. The syringes are plastic with a very tight-fitting rubber piston and curved tips that come to quite a fine point-so fine, in fact, that I have to shave a little of it off in order to get a big enough orifice for the Aerolite to come out at a reasonable rate. The syringes are easily cleaned with water after each use. Obviously, you use one syringe for glue, another for hardener and keep the third for casual mainlining when you glue the last wing panel on and realize you've left your 5/16-inch box wrench inside the wing.

Steve Wilkinson

From "Construction Notes" Falco Builders Letter, September 1987

Steve Wilkinson wants to know if he is the last Falco builder to discover that you should not use Stits Microputty or any other polyester resin fillers on a wood airplane. The stuff absorbs water. Polyester resin in hygroscopic and pulls more than its fair share of moisture out of the atmosphere. Steve scraped the filler off and said the wood felt damp.

From "Construction Notes" Falco Builders Letter, September 1987

In all of our praise of West System epoxy, we seem to have failed to mention one property of the resin that's important. Many epoxies used as wood glues or laminating resins do not sand well. Safe-t-poxy mixed with microballons, for example, makes a substance that is just a little too gummy to sand. Cured West System becomes a hard, brittle, block of plastic, and you can sand it without the sandpaper loading up. European builders take note: the only European distributor of West System epoxy is Wessex Resin, 189/193 Spring Road, Choling, Southamption SO27NY, England. They probably sell to boatyards all over Europe, so drop them a letter if you live in another country.

From "Construction Notes" Falco Builders Letter, December 1987

At various places in the Falco construction manual, we mention using Loctite to hold things in place while you drill. The bushings between the half-inch shafts and the universal joints of the retraction system are one such case. Loctite is an excellent adhesive, but what if you ever have to get the pieces apart? Falco builder Chuck Hubscher mentioned that his company uses Loctite for temporary attachments, and he let me in on a little secret. At around 300°F, Loctite let's go and turns to an oily substance which does not harden again when it cools. So whenever they need to get something apart, they just break out a propane torch.

From "Construction Notes" Falco Builders Letter, June 1986

Builders using Aerolite and Penacolite have sometimes asked about the dangers of using glues containing formaldehyde. I have not been aware of any problems, although builders have to be careful with Aerolite's formic acid which will irritate the skin. Because urea-formaldehyde adhesives are used so widely-chipboard, underlayment and some plywoods are made with urea-formaldehyde-the National Cancer Institute released an epidemiological study in March which concludes that low-level exposures of formaldehyde do not appear appear to increase the cancer risks in humans.

The study was conducted over a four-year period at a cost of $1.2 million. It traced the medical histories of 26,561 workers employed at 10 industrial facilities that produce or use formaldehyde, involving more than 600,000 man-years.

Formaldehyde, or HCHO, is a simple compound consisting of carbon, hydrogen and oxygen that is produced naturally by all plants, animals and humans. About nine billion pounds of formaldehyde are manufactured annually in the U.S. for a wide assortment of applications ranging from embalming fluid to permanent-press clothing to particleboard production. It is estimated that products containing formaldehyde or one of its derivatives account for 8 percent of the U.S. gross national product.

While it has long been known that formaldehyde vapor is an irritant to eyes, nose and throat, within the last decade the government has stepped up its risk assessment efforts to determine if it should be regulated as a carcinogen, and last May the EPA added formaldehyde to its list of "probable human carcinogens." This was based on a study in which rats developed nasal cancer when bombarded with fomaldehyde over extended periods. For more information, see the April 1986 issue of Wood & Wood Products.

From "Construction Notes" Falco Builders Letter, June 1986

As most of you know, I take a very conservative view of epoxy glues, although it appears that an epoxy-built Falco has no greater probability of glue failure than any of the fiberglass airplanes. I am coming to the conclusion that if you are going to use epoxy, the West System is probably the best choice. I am very impressed with this company. One of the things that I am impressed with is the testing that the company has done. So far most of the testing has been on moisture protection and fatigue. Jan Gougeon, who is in charge of their testing, has just begun some work on the higher temperature performance.

While Jan is not yet ready to put out the kind of temperature performance specifications that I would like, we do know from tests run by others that West System epoxies outperform T-88 and FPL-16A, but not by any wide margin. Gougeon's current testing is for creep at 95°F and near-ultimate loads. All of the flexible epoxies exhibit some creep, but the harder West System epoxies do not.

Jan actually agrees with my recommendations regarding epoxies, agreeing that it is difficult to recommend epoxies with their temperature performance being inferior to other proven glues. At the same time, neither of us are afraid of epoxy-built airplanes as long as they are painted white.

The Gougeon Brothers Co. has recently developed a new high-strength, high-density filler. They make a lot of wind turbine blades and the blades are fastened at the hub with steel "carrot studs", which are shaped like a carrot and glued into a matching hole. They have used asbestos as a high-strength gap-filler, but have been working on developing a replacement because of the health hazards of asbestos. The new filler is proprietary, but it is a talc-like powder which you mix with the epoxy.

The West System epoxies are a 100%-solids system. Thinners are added to get the epoxy to penetrate the wood fibers and still retain superior moisture protection, and these thinners become part of the chemical reaction and harden with the epoxy. They do not evaporate off like MEK or lacquer thinner; in fact, the use of such thinners has a disastrous effect on the moisture protection of the West System. Jan says they are working on improving the temperature performance of their epoxies, but it's a tough nut to crack. Every time they do all of the things to improve the temperature performance, other things start to go wrong which detract from the performance of the glue.

From "Construction Notes" Falco Builders Letter, June 1986

Richard Clements is using an epoxy called Bondmaster M666, which has been used on a lot of Steen Skybolt wings. Bondmaster M666 is made by National Starch & Chemical. I have called for technical information. The company's data sheet claims an aluminum lap shear test result of 2,000 psi at ­67°F, 2,500 psi at 77°F, 400 psi at 180°F and "not recommended" at 260°F. If true, this would be substantially better performance than T-88, but you should always conduct your own tests. (Spruce has a shear strength of 990 psi parallel to the grain at 15% moisture content.)

From "Tool Talk" Falco Builders Letter, September 1985

I'm probably the last person in the world to have discovered it, but the ideal dispenser for the formic-acid catalyst for Aerolite is one of those little wedge-shaped, sponge-like foam paint brushes sold in hardware stores as cheap substitutes for sash brushes. After using an ordinary brush for months-numerous hasty trips between bottle and wood to get enough acid on the wood quickly enough to keep it damp-my painter friend Jim Catalano, an ex-Pitts builder, suggested one of those sponge brushes. The head holds enough formic acid to wet most of a skin sheet, they're painfully cheap, and the handle is short enough that the brush just lives in the capped acid bottle-no need to squeeze out the little sponge every time you use it.

Steve Wilkinson

From "Mailbox" Falco Builders Letter, December 1999

Aerodux 500 resorcinal glue with Hardener 501 is available from Custom Pack Adhesives. Their phone number is 1-800-454-4583 and they have a web page at www.custompak.com. Aerodux comes in three versions with different curing speeds and temperature ranges. They presently only provide the medium or fast setting hardener, but when I spoke on the phone with them, they said they may consider providing the slow setting hardener in the future. According to the literature, it appears that medium Aerodux can be used in temperatures down to 59 degrees and fast Aerodux can be used in temperatures as low as 50 degrees. This glue is discussed in the December 1997 builders newsletter by Alan Powell.

Ron Strong

I have a box of Aerolite glue, from a former builder who lives in Florida. What was apparently once a powder is now rather a brick, as it wasn't in a watertight container. The guy I got it from says it's still good, and that I can grind it back down and still use it, but I prefer to rely on a more imformed source, so I'm putting the question to you. Also, is there a shelf life on the hardener?

Kerry Bedsworth

Easy answer. Throw the Aerolite in the trash. One day, you will take someone for a ride in your Falco, and their life will depend on the structural integrity of the glue joints. I would never consider using Aeroline powder in that state.

And, no, there's no shelf life for the hardener. It's an acid and can be stored for a very long time. Don't worry about the hardener.

Scoti

From "Construction Notes" Falco Builders Letter, September 2001

Kerry Bedsworth asks about what to do with Aerolite powder that has solidified into a solid block. He was advised to grind it up and use it. I think that's terrible advice, and I think the only responsible thing to do is to throw the glue away. My lord, you will one day take a member of your family for a ride in the Falco, and the idea of using glue that is marginal in any way is unthinkable.

From "Construction Notes" Falco Builders Letter, December 2001

Swiss Falco builder Andrej Cakmak asked about using Aerodux 185, a recommended glue in Switzerland for wood aircraft, and asked us to compare this to Aerodux 500. It's a difficult question to answer for the simple reason that I don't know the difference.

Andrej says, "After reading all I was able to find about these glues, Aerodux 185 belongs to the same family of resorcinol glues, but the resin is liquid and the hardener is in powder form. Aerodux 500 have both components liquid and quite inflammable with 'flash points' for one 31°C and the other 38°C -- consider keeping it in refrigerator! The mixing ratio is not the same -- Aerodux 500 is 1:1 and for 185 it's 5:1. The 185 is somehow faster drying (but not as fast as Aerolite 306), and less indicated for wood-ceramics or wood-porous surfaces gluing, but as they say extraordinary good with wood-wood glueing and water resistance.

I think that I am going to investigate a little more with Dynachem factory who seems to make both glues. In fact, I would use Aerolite 306 or Aerodux 500, but it seems very hard to find in France or Switzerland, and many English providers refuse to ship them, except as 'dangerous stuff'!"

Andrej Cakmak