COVERING PLYWOOD SURFACES
Covering a plywood surface with dope and fabric requires special attention. The surface must be properly prepared before the fabric is attached. If this is not done correctly there exists a possibility that the fabric will separate from the surface during flight. This applies to large plywood surfaces such as wings and control surfaces. Separation of fabric from the surface could have serious consequences.
To begin with, you should brush or spray at least 2 coats of epoxy varnish over the wooden surface. Allow that to fully cure for a period of about 1 week before proceeding. Use of spar varnish or one-part varnishes is not recommended. The solvents in the dope that will be applied will often lift a one-part varnish. If you are covering a surface that has already been prepared and you are unsure as to what is on the plywood, test it by rubbing MEK on it. If the varnish lifts it is a one-part product. If this is the case you should recoat the surface using epoxy varnish. Old varnishes can be recoated with a two-part epoxy varnish.
After the epoxy varnish has cured (6-7 days), lightly scuff sand the surface of the plywood using dry sandpaper. Then spray three full cross-coats of nitrate dope or a nitrate dope directly to the varnished wood. It will be necessary to thin the nitrate dope 1 part thinner to 1 part nitrate. Do not use butyrate dope for this step—the fabric will not adhere to the surface.
Allow the dope to dry for a short period of time and then attach the fabric. Ceconite light fabric is the best choice for a plywood surface. Cement this fabric to the outer edges of the surface you are covering using Super Seam cement. Avoid any wrinkles when you lay it in place. You will want to pull it somewhat taut during this step. You can use your iron at 225 degrees and heat form over any curved shapes before you cement the fabric in place.
After allowing the cement to dry for 1 hour or so, carefully heat shrink the fabric with as low a temperature as possible—225 degrees F. The objective is to get all of the wrinkles out without shrinking the fabric too much. If the fabric is shrunk too tight it will pull away from any low areas on the wooden skin. You do not want the fabric to pull away in any area because of the adhesion needed for the fabric throughout the surface.
Next, brush a very thin coat of non-tautening nitrate dope directly onto the fabric. Use the brush to press the fabric into the nitrate dope that is already in place on the plywood. Brush out all of the air bubbles. Again, do not use butyrate dope for this step. After brushing on a coat of nitrate dope you should then spray two more cross coats of nitrate onto the fabric. This will begin a normal build up. After this you can then begin the steps outlined for normal fabric covering including a buildup of clear butyrate, silver butyrate, and topcoat.
Old dope finishes can often be rejuvenated to a newer appearance. It is important that you ascertain what types of chemicals have been used throughout the process when your airplane was recovered. If it is not butyrate dope then don’t try to rejuvenate. (An exception to this is Poly-Fiber covering materials. If the aircraft is covered using the Poly-Fiber system it may be rejuvenated using their rejuvenator—not butyrate rejuvenator). You will be wasting your time trying to restore old paint that is enamel or something other than butyrate dope. You should have a FAA Form 337 in your aircraft records that will tell you the type of covering process that was used along with the chemicals that were used.
Dope finish on an aircraft will tend to fade and crack over a number of years. This is particularly true if the airplane is exposed to the weather regularly. What happens to the finish is that the plasticizer present in the dope will begin to migrate out of the film over a long period of time. We are talking 10 years plus on an airplane that is tied down outside. When the dope loses its plasticizer it will be brittle and will crack when pressed or punched. It usually cracks in the form of several circles. This cracking is termed “ringworm.” If this cracking and fading is not taken care of the underlying fabric will begin to deteriorate. Understand that rejuvenator will not fill cracks. It is a solvent with new plasticizers that will soak into the old coating and restore its flexibility.
Lets assume the fabric under the cracking dope has been tested and determined to be airworthy. The testing should be done using a fabric tester. If the fabric is good then the finish may be rejuvenated to provide an extended life for the entire covering on the airplane. This is an alternative to recovering the airplane and will provide a good surface for several more years.
Lets assume that you can rejuvenate the dope. Here is how we do it.
Rejuvenating dope is not a long term fix for cracking finishes. The process will, however, provide a lasting finish for a few years without having to completely recover the airplane. Remember, you must determine that the fabric beneath the cracked surface is good before starting the rejuvenating process. If the area in question shows signs of fabric deterioration due to exposure to the sun, etc. then rejuvenating should not be attempted. If the fabric has lost it’s strength then it is time to recover the airplane.
- Thoroughly wash the old fabric using a good cleaner such as the Poly-Fiber 310 Cleaner. Remove all of the dirt and oil from the surface of the area to be rejuvenated.
- Lightly wet sand the area using #400 wet or dry sandpaper.
- Completely flush the area with clear water.
- After the surface has dried, wipe with a clean cloth using Poly-Fiber’s Paint Cleaning Solvent C-2210. this will remove any remaining oil or contaminants.
- Tack rag the entire surface.
- Thin the rejuvenator according to directions. This is usually 40% rejuvenator to 60% butyrate thinner.
- Spray on three cross coats of the thinned rejuvenator over the surface. Wait at least 20 minutes between each coat. No sanding is required between coats.
- The rejuvenator will be very soft and wet for some period of time. Do not touch the surface. You may even notice a sagging of the fabric in the area being rejuvenated. This is normal. Let the surface dry for at least 12 hours.
- If small cracks are still visible, spray 1-2 cross coats of thinned butyrate silver dope to fill the cracks. It is actually recommended that you spray a couple of coats of silver over the area regardless. This will present a smoother surface and ensure protection from the sun.
- After the silver dope has thoroughly dried (2-3 days), reapply the butyrate color. Usually, 1-2 cross coats of butyrate color will provide adequate coverage.