Tony Bingelis •

USING ADHESIVES Parti

"Each has its own peculiarities, limitations, advantages and disadvantages . . . some more than others" You can save yourself a lot of time and needless worry by electing to use the glue recommended in your construction plans or construction manual for your project. To do otherwise could entail quite a bit of research and study on your part - unless, of course, you already have experience with a glue that you trust and like. In trying to decide what glue to use, it is only logical that you would first think of scouting around to see what types of glue you can buy locally, and thereby eliminate the cost and bother of importing it from another state. But what kind of glue would you be looking for? The only two aircraft quality glues that have been generally available through local hardware stores, lumber yards and some discount stores, are the plastic resin and resorcinol glues (Weldwood, Borden's, U. S. Plywood, etc.).

Unfortunately, these glues are becoming difficult to find in many localities, especially in quart can sizes. Although you can still find these glues, you had better be prepared to try a number of different outlets before you succeed. In place of the plastic resin and resorcinol glues, you are more likely to find store shelves and display racks jammed with all kinds of epoxies in twin syringes, vials and tubes that are formulated to be "super quick setting" and "indestructible". Still other tubes containing Silicone adhesives, Contact Cement and exotic metal cements are stacked in layers on wall hooks. All of these adhesives seem to have one thing in common. They are packaged in tiny, almost impossible to open, plastic packages . . . each with a similarly large price tag for a couple of teaspoons (more or less) of the stuff.

What is the best glue to use? For what? For wood? Fabric? Metal? Rubber? These are some of the adhesives left over from my last project. Noticeably absent are the Aerolite and Weldwood plastic resin glues . . . they were all used up.

I suppose that is good merchandising because most folks nowadays don't build much of anything and all they need is a few drops of a quick cure adhesive to repair their picture frame, mend a broken vase, or whatever. However, that doesn't help us. We need glue that is suitable for aircraft construction and we need it in larger quantities. Except for the plastic resin and resorcinol glues that you may still find locally, none of the other unproven glues, the "White Glues" and Elmer's Carpenter Wood Glue included, are considered to be suitable for the assembly of important aircraft structural components. Most builders, therefore, wisely limit their choice of glue to one of the adhesives sold by the suppliers advertising in SPORT AVIATION. These are established sources for the aviation quality plastic resin and resorcinol glues, as well as for a broad variety of other popular glues and epoxy adhesives generally considered to be suitable for aircraft construction. What Is The Best Glue To Use? I have used most of the better known glues and adhesives in at least a half dozen wood aircraft projects and, after thinking about it, have reached the following conclusions: 1. There is no single "best" glue. 2. All of the structural adhesives/ glues purchased from our regular homebuilt suppliers are good strong glues when mixed and used according to the manufacturer's instructions. 3. Each type of glue has its own peculiarities, limitations, advantages and disadvantages ... some more than others. Therefore, based on the experiences of other builders, and my own, I would

recommend that you consider the folSPORT AVIATION 33

Use the best aircraft quality adhesives you can buy for your project. You will be glad you did. (Boredom Fighter by J. Taylor, Round Rock, Texas.)

lowing factors carefully before you select a glue for your next project: 1. How serious a health hazard will

that glue present for me? 2. What kind of workshop temperatures and humidities will I have to maintain for the proper use of the glue? 3. How difficult will it be for me to accurately mix that particular glue formula

in small quantities? 4. How long a shelf life does the glue have? (A project can drag on for years.) 5. Does the glue have gap filling qualities or must I make near perfect joints even/time? 6. Cost? (Least important factor.) To further assist you in your selection of the best glue for your own project, I will detail and evaluate some of the essential features of the glues and adhesives most frequently used by homebuilders. Hopefully, you will find that these observations do contain more than a modicum of objectivity.

Plastic Resin Glues This is a favorite glue with many Pietenpol, Fly Baby and Volksplane builders . . . mine too ... for simple spruce to plywood joints.

The glue is a pre-catalyzed, urea formaldehyde adhesive in powder form. It cures chemically into a hard, strong material that is water resistant and resistant to the growth of fungi or bacteria . . . and it is not affected by oil, gasoline or paint solvents. The brand I have used the most (because of local availability) is Weldwood. Other brands of plastic resin glue (Borden's, U. S. Plywood, etc.) are just as good because they are also produced to the same requirements, Federal 34 OCTOBER 1988

Specification MMM-A-1886, Type II. Plastic resin glue is the least expensive structural glue you can buy that is suitable for aircraft construction. A one pound can currently sells for about $3.95. It is the only aircraft quality glue that does not need a catalyst or hardener because it is already precatalyzed. A lot of builders worry about shelf life (very important with epoxy adhesives) but I found that plastic resin glue is useable for as long as it maintains its powder form. The manufacturer, however, limits his warranty to a one year shelf life. When you buy plastic resin glue, shake the can to determine that the contents are still in powder form. If you can't detect that powder swishing sensation and instead hear a clunking sound, the contents are probably hardened and unuseable. Plastic resin glue powder is very sensitive to humidity so the lid of the can must always be replaced tightly after each use lest moisture be absorbed by the powder and cause it to become partially solidified. If the contents do develop hard lumps, the lumps must not be pulverized with a hammer in an attempt to salvage it ... the glue is ruined, and should be discarded. To use the blue, shake out the amount of resin powder you think you'll need into a plastic cup or glass jar. Add water (10 parts powder/6 parts water by weight), and stir until you have a uniform lump-free creamy mixture. Touch the mixture with your finger. If it feels sticky when you rub your thumb against it, the glue mixture is good. If it doesn't feel sticky, get rid of it.

Apply the glue to both of the bonding surfaces if delay in assembly is anticipated. Incidentally, the glue contains formaldehyde so it may cause an irritation to the skin. However, that's unlikely as I don't know anybody who was so affected by the glue. The glue makes a strong bond that will not fail at the glue line provided: 1. You make a good fitting joint. 2. You clamp the work evenly to develop a thin uniform glue line. 3. Your workshop temperature is at least 70 degrees F. and you keep it that way until it cures . . . at least 13 hours (only 8 hours required at 80 degrees). Squeezed out plastic resin glue that has cured may seem brittle because you can sometimes chip it away, however, this is not the case inside properly clamped, close fitting joints. It is an excellent glue for installing gussets and plywood skins, but is not as effective in ungussetted, cross-grain or end grain (butt) joints, and poorly fitted joints, because it lacks a gap filling capability. Nevertheless, the glue is as close to a fool proof and economical structural glue as can be found. Resorcinol Glue I get the impression that the U. S. Forest Products Laboratory has long considered resorcinol to be the Cadillac of glues. Some well known brand names include Weldwood, Borden's, U. S. Plywood and the Koppers Company's somewhat easier to use resorcinol, PenacoliteG-1131, is in the same class. Resorcinol glues are absolutely waterproof and are virtually immune to the effects of age, moisture or heat. For these attributes, however, you must pay the price. Resorcinol glues cost about as much as epoxies, and about five times more than the plastic resin glue . . . about $21/qt. Resorcinol glue is a two component adhesive consisting of a purplish liquid and a catalyst powder. Each is packaged in separate, (mostly) one pound size cans. Preparing the glue for use requires careful measurement, by weight preferably, of both the liquid and the powder catalyst. The recommended mix (Weldwood) is 4 parts resin to 1 part catalyst powder by weight. The glue will stain wood so you should make a

reasonable effort to keep the glue inside the joints where it belongs, and not smudged all over the structure. As for shelf life, the manufacturer says only that the powder and liquid components remain useable for a reasonable length of time when stored at temperatures of 70 degrees F. or

UDOK FOR SIGNS OF GUJE SQUEEZE-OUT (TYPICAL)

UNREINFORCED END (BUTT] JOINTS WILL FAIL AND ARE UNACCEPTABLE .

NAILS USED TO EXERT CLAMPING PRESSURE.

GUSSET OR PLYWOOD SKIN ON ONE SIDE .

REINFORCED GLUE JOINTS

GUSSETS BOTH SIDES (OR PLYWOOD SKIN ONE SIDE)

CLAMPS WILL DAMAGE SURFACES.

PROTECTIVE WOOD BLOCK UNDER C-CLAMPS .

UNEVEN CLAMPING PRESSURE

BETTER PRESSURE DISTRIBUTION

/ PROTECTIVE PADS

NAIL STRIPS USED TO APPLY PRESSURE TO GLUE JOINTS WHEREVER CLAMPS CAN'T REACH. WAX PAPER PLYWOOD SKIN

GLUE SQUEEZE-OUT (TYPICAL)

RIB ATTACHMENTS MADE BY NAILING THRU SPAR UPRIGHT OR BY MEANS OF

CORNER BLOCKS.

NAIL THIS HALF OF FORM BLOCK TO BENCH.

GLUING AND CLAMPING DETAILS

LAMINATING JIG

(FOR TAIL TIP BOWS, ETC.)

SPORT AVIATION 35

My Texas garden gate test laboratory. These wood test sample of glued joints are all cut-offs from my last project. Some have endured the Texas sun, rains and winds for up to 5 years. Samples were glued with resorcinol, T-88 and some with Safe-T-Poxy. All joints, varnished and bare survive in serviceable condition . . . weather beaten but serviceable.

below in tightly closed containers. Resorcinol glued assemblies exhibit a remarkable resistance to deterioration in strength with the passage of time. That is a track record other adhesives have not been able to match. Of course, most of the epoxy adhesives are relatively new and nobody can predict if they will one day challenge resorcinol's strength/longevity record. Resorcinal, as good as it is, is considered by many builders to be a difficult glue to use. It really is not. The activated glue is applied by brushing, usually, to both surfaces. After the surfaces have been mated, surface pressure should be applied as soon as possible. When pressure is applied, there should be evidence in the form of glue squeeze-out, indicating that a uniform adhesive contact is being achieved between the mating surfaces. This does require closely fitted joints and a moderate clamping pressure, evenly distributed, to develop the full strength in an assembly. Resorcinol is especially effective for laminating work and for the installation of plywood skins. Sufficient clamping pressures are easily and quickly obtained over large plywood covered areas with nailing strips. It is extremely important that the working temperature in your shop be at, or above, 70 degrees F. - and main-

tained at that temperature, or higher, until the glue has cured (at least 8 hours). The higher the temperature, the

quicker it will set. However, if your shop 36 OCTOBER 1988

temperature is much over 70 degrees F., your working time (pot life of the glue) will be noticeably reduced. That means some fast work may be necessary. The glue line must be still wet when the two parts are assembled. Double spreading will help assure this but you still have to work fast when working with large skins. The manufacturer cautions that the glue is a strong sensitizer and that skin contact should be avoided. Most builders don't seem to be experiencing any difficulties in this regard. Resorcinol glues appear to have an indefinite shelf life. Aerolite 306 During World War II, deHavilland assembled its Mosquito bombers with Aerolite glue (a product of the CibaGeigy Plastics and Additives Co., England). That, as well as the adhesive's worldwide reputation as being one of the best glues available, certainly places it at the top of my list of favorite

glues . . . especially for the assembly of wing ribs and smaller wood parts. Actually, even gluing larger skin areas is feasible if the slower GBP-X hardener is used instead of the fast setting GBQ-X. Aerolite, like plastic resin glue, is an urea-formaldehyde resin glue. Unlike plastic resin's tan color, this powder is pure white so don't get it mixed up with your plastic bag of micro balloons or Q cells. Aerolite powder must first be pre-

pared by mixing it with water in a ratio of approximately 2 parts water to one part Aerolite 306 (by weight). This converts the resin into a clear library-pastelike mixture that remains useable for as much as two weeks when stored in a glass jar with a lid. Aerolite resin must be activated with a hardener in what could be considered to be a tricky application ritual. That is, the Aerolite resin ("paste") is first applied to one side of the wood joint with a brush or a spatula. Next, the other side is then quickly wetted with the liquid hardener. The two wood parts must then be brought together before the hardener evaporates or dries. The best way to apply the hardener is with a cheap throw-away foam brush. Its sponge-like characteristics permit it to hold more fluid than a bristle brush. Usually, the resin is applied to the wood and the liquid hardener to the denser surface of the plywood. The biggest risk in using Aerolite is to inadvertently forget to apply that clear liquid hardener to one side of the joint. Forget that hardener, and your joint will have been assembled with the equivalent strength of library paste. Because the hardener liquid is clear, it is hard to tell where it has been applied. It, therefore, can be inadvertently overlooked. To make its presence more noticeable, builders often added a few drops of cake coloring to the hardener so that it would show up as soon as it was applied. Now, I understand that the hardener can be purchased color coded (in England, anyway) to reflect what strength (speed) hardener is being used. Aerolite has good gap filling qualities and perfectly fitted joints are not as important as with some other glues. Builders living in colder climates find that the glue's lower working temperatures (as low as 50 degrees F.) are a great incentive to working year round on their projects. As for shelf life, the Aerolite 306 powder should be good for at least 2 years and the hardener for 3 years. Best of all, you can keep a small jar of the mixed Aerolite powder/water resin on your workshop bench ready for instant use . . . it will keep for about 2 weeks. This is a great convenience when you have a number of small jobs to glue dur-

ing the week. The Aerolite resin is considered to be harmless to the user although skin contact should be avoided with the hardener. More next month. If you wish to contact the author for additional information, please write to Tony Bingelis, 8509 Greenflint Lane, Austin, TX 78759.