Topcoats For Fabric Airplanes  By Ron Alexander

If fabric covering is fairly easy to do why do you see so many problems with the fabric and coatings on aircraft? Surely there is more to this than meets the eye. I know you have seen airplanes with the coatings literally peeling off in sheets. We have all seen an Aeronca Champ or a J-3 Cub with wing tips that are bowed up (not a new type of high-lift device). What causes these problems and all of the other fabric problems so many people encounter? Is it dependent upon the type of covering process they are using? Rarely is that the case. Each type of fabric covering process has its own advantages.

One important point, regardless of the type of fabric covering process you select follow the instructions to the letter. Do not experiment with your own techniques or mix fabric covering systems. As an example, do not use Grade A tapes on Ceconite fabric or nitrate dope on Poly-Fiber fabric. These are common practices that cause problems.

There are seven major problems encountered in covering aircraft. This list incorporates the most common reasons aircraft owners have to recover their airplanes prematurely. The problems are:

  1. Inadequate preparation of surfaces
  2. Selecting the wrong weight of fabric
  3. Not following the procedure manual
  4. Improper tautness of the fabric
  5. The 1st chemical coat improperly applied
  6. Inadequate protection from the UV rays of the sun
  7. Topcoat problems
Prior to probing into these specific problem areas, it is important to note that there are a few general dos and don’ts to remember. The first is not to rush the covering process. Many builders push to complete an aircraft for an airshow or convention (I am sure you have never done this). Rushing through the fabric covering stage is not conducive to completing a trophy-winning airplane. Fabric covering involves the spraying of several coats of chemicals. Each coat must thoroughly dry before the next one can be applied. A good rule of thumb is to do one coat in the morning followed by one coat in the afternoon. Allow plenty of time for each coat to dry. Another mistake made by many first time coverers is to tackle a large surface initially. Do not start covering a wing or fuselage without practicing. Start with a practice panel and then proceed to a small control surface.

Another generic problem area is found with temperature and humidity. The ideal temperature for fabric work is 77 degrees F with humidity between 0% and 70%. If the temperature is below 65 degrees and/or the humidity above 80% wait until a better day.

Inadequate Preparation of Surfaces

Often a builder or aircraft restorer will have the mistaken idea that fabric covering begins when you cement the fabric in place. This is far from reality. As a matter of fact, a lot of time and effort will be needed prior to ever cutting the fabric for placement on the aircraft. Anyone who has ever restored an airplane certainly knows that most of the total work involved is in the preparation phase. You should ensure that the surfaces are in good condition with needed repairs accomplished and that they are properly varnished or primed. That means using an epoxy varnish or primer.

All of the sharp edges that could potentially cut the fabric should be covered with anti-chafe tape. This usually involves rivet heads, metal seams, and sharp edges. Let your sense of touch be your guide. If you feel something sharp cover it with the anti-chafe tape. Do not use masking tape for this purpose. It will retain moisture and cause problems later on. Also, paper masking tape will turn brown with age and possibly show through a light colored paint.

Selecting the Correct Fabric

This decision has a major impact upon the life of the fabric on your aircraft. If you use a fabric too light in weight for your airplane you will certainly have associated problems. Selection of fabric for your airplane is simple. If you are covering an ultralight, glider, or an airplane with a small horsepower engine (below 65 HP), you can safely use lightweight fabric. If you are covering an aerobatic aircraft, large bi-plane, warbird or crop duster, in short any high wing loading aircraft operating under severe conditions use heavy-duty fabric. All other airplanes (this constitutes the majority) should use the medium weight fabric. You cannot go wrong using a heavier weight fabric. Problems are much more likely to occur when you use a lighter weight fabric than the one recommended for your airplane. These problems surface in the form of damaged coatings on top of the fabric caused by movement of the fabric itself in the form of drumming or beating. So, if in doubt, add a few pounds and go with a heavier fabric. The Ceconite designations for different weight fabrics are as follows:

C101 Heavy Weight

C102 Medium Weight

Uncertified light Light Weight

Follow the Instructions

This is a key point. You must follow the instructions written by the fabric process manufacturer. Reviewing the STC’s mentioned earlier, on a production airplane you cannot mix fabric covering processes. Not only is this a legal issue regarding the airworthiness of your airplane but it also can result in having to recover your airplane prematurely. In other words, do what the manual tells you to do and use the correct products.

Another area of concern is experimenting with a proven system. Listening to so-called “experts” who provide you with a quick and easy way to short-circuit the covering process. I have encountered individuals who are experimenting with various steps of the covering process from the type of fabric cement they use to using latex house paint as a final topcoat. More on topcoats later.

Fabric Tautness

How tight should the fabric be before you start putting on the coatings? This is a very good question and an additional problem area. If the fabric tautness is not correct you may have difficulties. If the fabric is too tight you may do damage to the structure itself. If too loose, you may have the coatings crack and peel due to too much movement. In any case, take the time to properly shrink the fabric. Remember, the fabric is capable of shrinking about 10%. Keep that in mind when you cement the fabric in place. You should initially put it on the surface so that it is snug—a technical term meaning neither too loose nor too tight. A good rule of thumb—on a surface such as a wing you should be able to lift the fabric about 1 inch above a rib prior to shrinking.

One thing is certain; you must use an iron to shrink fabric. If you have a heat gun hide it until you have covered the airplane. You cannot control the temperature emitted from a heat gun. It is absolutely essential that you shrink the fabric using an accurate temperature from the iron. This can only be achieved using an iron that is rated at 1100 watts or higher. You must calibrate this iron for accuracy. When you shrink the fabric it must be done at a precise temperature within + or – 10 degrees. The initial shrinking of the fabric is done at 250 degrees F. At this temperature the fabric will shrink about 5%. If you are using nitrate and butyrate dopes this is the final temperature that you will use. Nitrate and butyrate dope (yes, even non-tautening) will continue to shrink fabric over a long period of time. If you have an ultralight or smaller aircraft you may want to stop shrinking at 250 because of structural considerations. Fabric, when being shrunk, has incredible power to bend and twist. You should watch for evidence of this while using the iron.

First Chemical Coat

After you have attached the fabric to the surfaces of the airplane and tightened it properly, you are ready to apply the first coat of chemical. This process is generally referred to as “sealing the fabric”. The first coat must bond to the fabric or all subsequent coats are in jeopardy of peeling. I am sure many of you have witnessed first hand the peeling off of fabric coatings. This is a common problem area. If the first coat does not bond to the fabric you can literally peel off all chemical coats. I have seen aircraft that you could take an air gun and blow the coatings off of the fabric in sheets. The problem is usually caused by one of three things: (1) the fabric was contaminated with dirt or oil, (2) the chemical was not applied properly, or (3) the chemical was applied in temperatures that were too cold.

The first chemical coat also provides a secondary function; it acts as cement that soaks through the fabric and further acts to secure the fabric to the airframe. I recommend brushing the first coat onto the fabric. This should be done only after you have thoroughly cleaned the fabric using a cotton rag and reducer or MEK. All traces of oil and dirt must be removed. A tack rag should be used to wipe down the fabric just prior to applying the first coat. Failure to remove dirt and oil will result in an inadequate bond.

With the first coat we are working to literally “encapsulate” the fibers of the fabric. This means the chemical should flow under the fibers and “grip them” for a good bond. Polyester fabric does not absorb liquid. This is why I recommend brushing on the first coat. You must penetrate the fabric with the liquid to get it to the backside. Unless you have experience covering aircraft, it is difficult to tell when you have an adequate penetration if you are spraying this first coat. Use a high quality polyester or natural-bristle brush. Brush on a wet coat. Be sure the chemical is penetrating the fabric and flowing to the underneath side. You must be careful not to allow it to drip through to the other side of the surface. To do so may show up on the final color coats as a difference in gloss. You also need to be careful to work quickly and not leave any brush marks. In normal temperatures the coating will dry rapidly. If you are using nitrate dope, you should avoid flooding it to the inside of the fabric thereby creating runs. They may show on the final finish. By the way, if you are using nitrate and butyrate dopes it is imperative that you use only nitrate products for the first chemical coat. The ones available are Classic Prep, Rand-O-Proof, and regular nitrate dope. Butyrate dope will not stick to bare polyester fabrics.

Finally, take your time during this initial step. Unless you thoroughly encapsulate the fabric with the chemical you will be faced with this common fabric-covering problem.