In a previous article I discussed common fabric covering problems. You may recall that I listed the most common problems people encounter when covering an airplane with fabric.
As a review they are:
- Inadequate preparation of surfaces
- Selecting the wrong weight of fabric
- Not following the procedure manual
- Improper tautness of the fabric
- The 1st chemical coat improperly applied
- Inadequate protection from the UV rays of the sun
- Topcoat problems
The first 5 were previously discussed. Let’s continue with the last two in this article.
Inadequate Protection of Fabric from Sunlight
If we were to find one fabric covering problem that is the most common it would be this particular one. In spite of being touted as a lifetime fabric, polyesters will deteriorate. Their biggest enemy is sunlight. Specifically, the ultra-violet rays of the sun. Polyester fabric, when exposed to prolonged sunlight unprotected, will weaken and degenerate. The only thing protecting the fabric from the rays of the sun is some sort of UV blocking chemical. The best chemical to place in fabric coatings is aluminum pigment. Aluminum pigment suspended in a coating will protect the fabric. Of course, the correct number of coats must be applied according to the manufacturer’s recommendation. That will usually be 3 or more “cross coats”. A cross coat is one sprayed coat passing the spray gun north and south followed by a coat passing the gun east and west. In other words two perpendicular coats.
A common issue involving aluminum pigment is the settling of the pigment in the can. Aluminum pigment is heavy and will settle to the bottom of a can. It must be thoroughly and completely mixed prior to spraying. This is very important. If not mixed completely, you can be sure you will be recovering the airplane with new fabric within a short period of time, particularly if it is regularly exposed to the sun. Use a paint shaker at your local hardware store for best results. Even then, be sure the pigment is in suspension before you spray it.
Aluminum coats are sprayed onto the surface of the fabric. Most fabric systems suggest sanding occasionally between coats. This will provide a much smoother final surface. Be careful not to sand off all of the aluminum pigment. This is often mistakenly done. In other words, the proper number of silver coats is applied only to be sanded off with the end result being inadequate UV protection. By the way, you will often hear the aluminum coats referred to as the “silver coats”. This terminology has been in place for many years and is used to describe the aluminum pigment coats.
How do you know if you have sanded too much? A good test is to use a 60-watt light bulb held up to the fabric. Look through a cut out inspection hole and see if the light is blocked. If you can see light you need more silver coats. (A word of caution: Use only a caged and protected light bulb and do not place the light bulb inside a wing or fuselage that is full of solvent fumes. An accidental break of the bulb could ignite the fumes). If you are considering the purchase of a fabric covered airplane and you are unsure about the condition of the fabric, perform the test outlined above. If there is inadequate UV protection, you can often open an inspection plate, look through it to the top of the wing and actually see daylight. If you see light of any sort coming through the surface I would recommend you not buy the airplane. Chances are it has inadequate silver coats and the fabric will deteriorate prematurely. You will often see light along the edges of tapes where they have been sanded. This should not present a problem.
To recap, you must protect all fabric from sunlight. Be sure to put an adequate amount of UV protection in the form of at least 3 cross coats of the recommended chemical. Be sure to mix the pigment thoroughly prior to application. Do not sand off all of the protection. If you are sanding thoroughly between coats, you may want to apply 1-2 more coats than recommended. Remember the basic rules of spraying—spray out of the sunlight, out of the wind, and at the proper temperature and humidity.
The aluminum pigment that is used for the Poly-Fiber system is called “Poly-Spray”. For nitrate and butyrate dopes you can mix aluminum powder into butyrate dope or purchase a Randolph product called “Rand-O-Fill”.
Selection and application of the topcoat constitutes a major problem area. First of all, use the topcoat recommended by the fabric system manufacturer. Do not experiment with latex house paints and paints designed for metal. Yes, latex house paint. I have personally talked with a few builders who are emphatic about using latex house paint on their fabric covered airplane. If you want to recover your airplane within a year or two, then use paint designed for your garage. I am serious about this issue. Do not try to save money on your project at this stage. In the days when I owned a major supply company, our sales people fielded at least one call per week from a builder who had used a topcoat paint other than the one recommended by the fabric system manual. They wanted to know what could be done about their fabric—the paint was peeling off. Unfortunately, there is usually little that can be done to solve a major topcoat problem other than recovering the airplane. I have seen absolutely beautiful fabric airplanes, covered less than 1 year, with major topcoat cracking problems. When the owner is asked about the topcoat in 90% of the cases a product other than the one recommended has been used.
Just because a paint works well on metal does not necessarily mean it will work on fabric. Why? Because fabric flexes and moves during flight. The topcoat paint must also be flexible. Several paint manufacturers have designed finish coatings specifically for use on fabric. They have added chemicals that allow them to be flexible. If these additives are not present the paint will eventually crack due to the movement of the fabric. So, do not use enamels, lacquers, or epoxy paint over fabric. Use only polyurethane paints with the additives necessary for use on fabric. At the risk of being repetitious, use only the topcoat paint recommended by the fabric system’s manual.
What other topcoat problems might you encounter? There are several and I will discuss only the most prevalent.
The problem areas presented are the most common. Certainly other problems may be encountered depending upon the type of covering system being used. For instance, with nitrate and butyrate dope blushing may be a problem. This is caused by the temperature and/or humidity being too high. Dope roping is a common problem. Dopes are viscous materials and should be properly thinned prior to application. Improper brushing will contribute to this problem. Using too much air pressure in a spray gun often causes pinholes and bubbles. Fish eyeing may present itself due to contaminants on the surface of the fabric such as oil, wax, etc.
Fabric covering is not a difficult task. As a matter of fact, it is really quite simple and enjoyable. This is the one area a builder or restorer should enjoy doing. Learning the basics will keep you from encountering the problems we have discussed. Attend an EAA/SportAir fabric-covering workshop. Talk to other people who have experience with fabric covering. Of course, once again, read the manual thoroughly. Start covering with a small control surface so you can correct initial mistakes. You certainly have the potential to do an excellent job in this final step of building or restoring.
- Not getting the correct color match. This problem usually surfaces for one of two reasons. First of all, not shaking the paint is usually the culprit. You must shake the paint thoroughly within 24 hours of use. This is very important on color coats. Take the paint to your local hardware store and have them place it on the paint shaker. After doing this you can use the paint by simply stirring for about 2 weeks. The second part of this problem involves different paint batches. All paint manufacturers produce a paint color that may be just slightly different in shade from one batch to the next. You can solve this problem by buying total amount of paint necessary for your aircraft, opening all of the gallon cans and placing them in one large container (a clean plastic garbage can will work). After doing this, thoroughly mix the paint and then pour it back into the 1-gallon containers. You now should have one constant color shade in all of the cans.
- Bleed through of primers. If you use a dark green primer you may have difficulty hiding the color. I would recommend you only use a white primer. If you want a zinc chromate look on unpainted areas, use a dark green primer there but not on surfaces that you are going to paint a different color.
- Bleed through of other colors-silver coats as an example. Often it is difficult to hide the silver coats. This is particularly true when using a red or yellow color. The best solution to this problem is to spray 1-2 coats of white over the silver. This will provide a good foundation for subsequent color coats. Doing this will provide a more even final color and also require fewer gallons of the color coat. This can be important when using a red final finish. Red paints are typically much more expensive than other colors.
- Dust and dirt in final finish. To avoid this problem you must have a small spray booth. As discussed in other articles, this can be homemade and simple. You also need to filter the paint as you pour it from the can into the spray cup. Finally, clean the surface using a tack cloth just prior to spraying.
- Spraying problems. Of course, this is probably one of the more common difficulties incurred by individuals. This is a subject in and of itself. I will only say that you need to practice, practice, and practice, prior to spraying final topcoats. You need to have an adequate place to spray, correct temperatures, enough light, low humidity, etc. Proper equipment is absolutely essential.