Article - Sealing Fabric
After tightening the Ceconite fabric with heat, the next step in covering your airplane using nitrate and butyrate dope is to seal the fabric weave. This step is critical to a long-lasting cover job. It is very important that the first chemical coat be properly applied. This first coat provides the primary chemical bond to the fabric itself. All subsequent coats adhere to it.
Nitrate dope is applied as the first coat on all polyester fabric. Even though nitrate dope is very flammable, it must be used initially. Butyrate dope simply will not adhere to polyester fabric. Otherwise it would be used as the first coat. So, apply nitrate dope first and then all remaining coats will be butyrate to reduce the flammability issue.
The first coat of nitrate dope must bond to the fabric or all layers 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 and comprises one major reason that aircraft must be prematurely recovered. If the first coat does not bond to the fabric you can literally peel it off along with all subsequent 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 trick is to totally encapsulate the fabric without flooding it. Applying too much dope will cause runs to occur on the inside of the fabric that will probably show through the final coats. Years ago when Grade A Cotton fabric was used, it would actually absorb the dope. This is not true for polyester fabric. As an example, if you wipe up water using a cotton cloth you will see that it immediately absorbs the water. Try to wipe up water using polyester fabric -- no absorption at all. That means you must penetrate through to the underneath side of the fabric with the dope without putting it on so heavy that it runs underneath.
Only non-tautening nitrate dope should be used on your airplane. As the dopes shrink they tighten the fabric they have encapsulated. Non-tautening dopes will shrink as they age but to a lesser degree than regular dopes. When using this process you will not shrink the fabric to its maximum prior to applying the dopes. You must allow for the resulting shrinkage caused by the dopes themselves.
I recommend brushing the first coat of dope onto the fabric. This should be done only after you have thoroughly cleaned the fabric using a clean cotton rag and reducer or MEK. All traces of oil and dirt must be removed. A tack rag should then 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. As previously mentioned, polyester fabric does not readily absorb liquid. Care must be taken to ensure proper penetration of the nitrate dope through the fabric. This is why I recommend brushing on the first coat. You must penetrate the fabric with the liquid to get it to the backside or the underneath side of the surface. Unless you have experience covering aircraft, it is difficult to tell when you have an adequate penetration if you are spraying this first coat. Understand that it may be sprayed, but only if you are an experienced fabric person.
If you choose non-tautening nitrate dope as the first coat, then you will thin the dope 50-50 using nitrate thinner. In other words, use 1 gallon of thinner for each gallon of non-tautening nitrate dope. This step is essential. If the dope is not thinned properly it will not penetrate the fabric. Mix the two thoroughly. Then use a high quality bristle brush and brush on the first coat. Be very careful not to let the dope drip through to the bottom side of the fabric. Even though you will want to totally encapsulate, you do not want it to run through on the underneath side.
Brush on a wet coat. Be sure the chemical is penetrating the fabric and flowing to the underneath side. The fabric should appear glossy and wet. Do not leave any dry areas. 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. The basic idea of this step is to thoroughly brush on a wet coat and then make only one more pass with your brush to level out any small bubbles or marks that may have formed.
You will apply 3 coats of nitrate dope. The first is brushed on and the remaining coats are then sprayed onto the surface. Use a good quality spray gun and allow each coat to dry at least 30 minutes before applying succeeding coats. When you spray be sure you use the cross coat method. Apply one coat north and south and then another coat east and west -- this is one cross coat.
When applying the nitrate dope, be sure the temperature and humidity are favorable. You can't always find the perfect environment but do the best you can. Do not apply the dope if the temperature is less than 65 degrees F or higher than 85 degrees F. The humidity should be less than 50% if at all possible. Remember that nitrate dope is highly flammable. Do not use space heaters or any type of open flame to heat a workshop area. Store the nitrate dope in a safe place. Don't use electric drills to mix the dope and thinners.
Rand-O-Proof is another non-tautening nitrate dope. It is a Randolph product designed to be used for the first chemical coats on Ceconite fabric. Rand-O-Proof should be thinned 1 to 1 just like nitrate dope. It is also flammable and sensitive to temperatures like nitrate dope. Again, three coats are recommended. The first is brushed on and then the next two sprayed.
If you are covering a plywood surface, this first coat is doubly important. Before placing the fabric onto a plywood structure, you should have sprayed on at least 3 coats of nitrate dope or Rand-O-Proof onto the plywood itself as a pre-coat. This is very important to achieve proper bonding of the fabric to the plywood. This procedure also ensures that the fabric will be entirely encapsulated. Never use butyrate dope for this step. It will provide no adhesion.
After providing this pre-coat, the first coat of nitrate dope or Rand-O-Proof will be brushed onto the surface of the fabric. The solvents in this coat will reactivate the dope that was pre-coated. This will cause the pre-coated dope to adhere to the fabric and penetrate the backside of the fabric. A very thin coat is brushed on initially. You should work very slowly and use the brush as a tool to literally press the fabric into the drying nitrate product. This will achieve the necessary adhesion of the fabric to the plywood structure.
Again, the important thing to remember is to penetrate and encapsulate the
Ceconite fabric with the first chemical coat. If the dope does not penetrate but rather lies on top of the fabric, you can be guaranteed that within 4-5 years it will peel off. What is the solution if you have all of the coatings peeling off of your airplane? Usually the fabric is good. If so, and you can easily peel coatings to bare fabric, you can often solve the problem. If the fabric has not been exposed to the sun or lost its strength in any other way, you can reapply the first coat of the process and continue the build-up of the coatings. Seek the advice of someone with fabric experience prior to doing this. If you cannot easily remove all coatings to reach bare fabric, do not try to use a paint stripper. Use of a paint stripper to remove chemicals from fabric is not a recommended procedure. The stripper itself is very difficult to remove from the fabric. The best solution is to apply the first coat of dope properly in the beginning.