TECH TIP: Wood Glues and Finishing Wood Surfaces
Certainly, the type of glue used in wood construction is of the utmost importance. Glue is the material used almost exclusively as a means of joining wood in aircraft construction. A component part is considered joined satisfactorily if the strength of the glue joint is approximately equal to the strength of the wood itself. The Aircraft Woodwork Technical Manual states “A strong glue joint is characterized by complete contact of glue and wood surfaces over the entire joint area, with a continuous film of glue between the wood layers, unbroken by air bubbles or foreign particles. The details of the gluing operation control the result.”
Wood surfaces must be clean (free of oil, grease, varnish, paint, etc.) prior to gluing. It is best not to sand the pieces prior to assembly. Sanding dust will often fill the pores of the wood resulting in a weak bond. Apply glue to both pieces that are to be joined and then place them together. After joining the wood pieces together you will want to apply pressure. The amount of clamping pressure does vary depending upon the type of glue to be used and the type of wood. Pressure is applied in order to distribute the load from the point of contact to other parts not directly under the load. Pressure can be applied using clamps or brass coated nails. As a rule of thumb, use about 4 nails per square inch of wood. Remember that the purpose of the nails is simply to hold the pieces together until the glue dries. As a general rule of thumb, a lighter pressure will be used with thin glue and a corresponding higher pressure used with thicker glue. It should be noted that using epoxy adhesives requires minimal clamping pressure. Application of too much pressure will starve the glue joint through excessive squeezing out of the glue itself.
If you are unsure about how much pressure to apply you should make up test samples. After allowing the glue to properly dry in the sample pieces, place the piece in a vise and try to break the wood. The wood itself should fail prior to the glue joint. Once you have made this determination you can then use the same gluing techniques on your structural pieces. It is also advisable to glue together 2-3 sample pieces. You can present one sample to the FAA inspector and keep another sample for several years (10-20) to test again at that time. That will help you determine how the glue is holding up inside your airplane. Again, AC43-13 has detailed information on the proper application of pressure during wood gluing.
Types of Glue
Now we enter the world of controversy. There are as many opinions concerning what type of wood glue to use, as there are builders. Basically, the types of glue can be broken down into the following categories: Casein glues, plastic resin glues, resorcinol glues, epoxy glues, and polyurethane glues. One very important note regarding your choice of adhesives, use the type of glue recommended by the kit manufacturer if you are building a kit aircraft. Be sure to use any type of glue strictly in accordance with the instructions of the glue manufacturer. (An in-depth discussion of aircraft adhesives can be found in the July, 1996 edition of the EAA Experimenter.)
Casein glue was used in aircraft during the early 1900’s. It remained in use until about 1940. It was difficult to mix and slow to dry. It was not waterproof and could not withstand higher temperatures. Do not use casein glue on your aircraft. The new AC43-13 states “Casein adhesives should be considered obsolete for all repairs.”
Plastic Resin Glues
Plastic resin glue has been used in aircraft for many years. It is powdered glue that is mixed with water prior to use. It uses urea-formaldehyde to promote a chemical reaction. This type of glue is not waterproof but does have much better water resistance than casein glue. However, plastic resin glue has been shown to deteriorate in hot, moist environments. It is a fact that on a hot day an airplane parked on an asphalt ramp can experience temperatures in the inside of a wing that exceed 200 degrees F. It has been determined that at temperatures of 120 degrees F, urea-formaldehyde glues begin to deteriorate. Prolonged exposure to heat has a cumulative effect on the glue. AC43-13 now contains a warning concerning the use of plastic resin glues. It basically states that you should consider this type of glue obsolete for aircraft wood repairs and any proposed use should be discussed with the appropriate FAA office prior to use on a certificated aircraft.
At approximately the same time plastic resin glues were introduced resorcinol glue was manufactured. These glues consist of a 2-part mixture that is very thick and also is a dark purple color. The glue consists of a resin and a hardener. Because of its color it is easily recognized after being applied on aircraft structures. Proper mixing and recommended clamping pressure are both critical to achieving proper bonding strength. Aircraft builders and restorers have used resorcinol glues with success for a number of years. Some find the dark color of the glue objectionable. It does retain this dark color after it has dried on the wood. This type of glue is readily available from your aircraft supply company and can be used with success.
There are many epoxy glues on the market today. A popular glue used by many amateur builders is called T-88 and it is manufactured by System Three, Inc. Epoxy adhesives consist of a resin and a hardener that are mixed together just prior to use. The pot life (working time) of epoxies varies with the type of adhesive and with the temperature where you are working. Generally, you will have 30 minutes or more to complete the gluing process before the epoxy begins to gel. Curing time to maximum strength also varies. It is very important that you mix epoxy adhesives according to the instructions. Do not vary the amount of hardener to resin ratio. To do so will adversely affect the strength of the bond.
Fast curing epoxies, often referred to as 5-minute epoxies, are occasionally used to hold pieces together for further bonding. Do not use these epoxies for anything structural. They do not develop the necessary strength.
There exists some controversy concerning the use of epoxy adhesives on aircraft wood structures. There is a possible problem with high temperatures. I consulted Kern Hendricks of System Three, Inc. and he stated that he is not aware of any documented failures due to heat in any aircraft built with T-88 epoxy adhesive. He did say that if you are bonding wood at temperatures below 60 degrees you should allow the bond to cure at temperatures above 70 degrees for at least 24 hours prior to use. He also said, “Like all epoxy adhesives minimal clamping pressure should be used to avoid starving the glue joint through excessive squeeze out.
So, what is the bottom line concerning glues? A number of builders and restorers are using either resorcinol glue or one of the many epoxy glues available. T-88 and FPL-16A are two examples. Remember to protect your skin if you are using epoxies. No matter which type of glue you decide to use, test several samples. Try to destroy the test pieces after the glue has cured. You should destroy the wood prior to the glue joint failing. Refer to the aircraft maintenance or repair manual for specific instructions on acceptable glue selection for use on your aircraft. AC43.13-1B states “Adhesives meeting the requirements of a Mil Spec, Aerospace Material Specification, or Technical Standard Order (TSO) for wooden aircraft structures are satisfactory providing they are found to be compatible with existing structural materials in the aircraft and the fabrication methods to be used in the repair.”
Remember, when applying glues spread a thin, even layer on both surfaces to be joined. Be sure to read and follow the manufacturer’s application instructions.
Proper finishing of the wooden parts you have constructed is very important. The wood must be sealed and protected from moisture. It must also be protected from any chemicals that might permeate fabric during the covering process. The best types of varnish to use are epoxy varnishes and polyurethane varnishes. After proper curing, either of these will seal the wood and protect it from most chemicals. There are other varnishes available but they usually do not offer protection from chemicals. The added cost of an epoxy varnish or a polyurethane is insignificant compared to the protection they will offer.
Both of these varnishes are two-part mixtures. Hardware store polyurethane varnishes are usually not true polyurethanes. The varnish must be catalyzed to be considered a true polyurethane. Polyurethane varnish is recommended if you are exposing the wood to direct sunlight. Otherwise, epoxy varnish is the choice.
If you are applying varnish to an old surface, remove as much of the old varnish as possible by sanding. You need not remove all of the varnish unless you are using a polyurethane varnish. If so, the polyurethane will often act as a paint stripper on the old varnish. Usually, you can simply remove the flaking pieces, thoroughly clean the surface, and apply a coat of epoxy varnish.
On new wood, you must ensure the surfaces are free of grease, oil, and all contaminants. Remove any excess glue. Fill all holes using a wood filler such as SuperFil. Clean all enclosed spaces of sawdust, wood chips, etc. If you have marked the wood using any marker other than a pencil, remove the marking.
On interior surfaces that fabric will not contact, one heavy coat of epoxy varnish should be sufficient. It can be reduced and sprayed or reduced and brushed on the wood. The resulting film will, of course, be smoother if the varnish is sprayed. Proper thinning is important. Follow the directions. Spraying an assembled wing will be much easier than brushing. Large, broad surfaces will be easy to brush. The penetration into the wood is essentially the same if proper thinning is accomplished. The solvents cause the varnish to soak into the wood.
Pay particular attention to the end grain surfaces. They must be sealed and protected and are often overlooked. Also, if you are covering large wooden surfaces—such as a wing—with fabric, be sure to properly prepare the varnished surface according to the fabric covering procedure. This is of the utmost importance. Follow the manual for the covering system you select.
If you decide to use an epoxy glue please work with your A & P and IA prior to use to see if they will accept the brand you are using. Not all epoxy glues are equal – some are more widely used than others. Please review the chapter on wood glues in AC 43.13 for additional information.