Sewn-in repairs are often used instead of doped-in repairs. If you are going to do any sewing at all, you must use Ceconite hand sewing thread for the repair. You will need to purchase a 3-inch or 6-inch curved needle. That will save you time and frustration during the repair. The first step is the same as doing a doped-in repair. You must remove all coats of dope down to the layers of nitrate dope. The butyrate dope coats will usually consist of a few clear coats followed by the silver coats and then the color coats. All of these must be removed to effectively patch the fabric. Again, nitrate dope will not adhere to butyrateóthus the need to remove all of the butyrate dope. Removal of the dope is the same as outlined during the doped-in repair process.
After the dopes are removed down to the nitrate layer, you are then ready to begin the repair. Advisory Circular 43-13-1B outlines the type of stitch that needs to be used. It is basically a common baseball stitch. If the damage is small enough, you may be able to simply sew the torn area together using this stitch. If the damage is such that you cannot close the areas together by stitching (a large hole as an example), then you will have to sew in a new fabric section. If this is the case, cut the patch you will use out of a new piece of unshrunken, smooth fabric. In this instance we will use straight scissors to cut the fabric so pinked edges will not appear under the finishing tape. (Remember, pinking was used for more gluing area in the doped-in process). Attach this piece of fabric neatly by sewing it in place using the baseball stitch.
If you have used another piece of fabric to affect the repair, you must then calibrate an iron to 250 degrees F and shrink the new fabric.
The remaining steps are the same as outlined earlier for a doped-in repair. Apply finishing tapes over the sewn seams and rebuild the layers of nitrate and butyrate dope.
REPAIRING MAJOR DAMAGE
As previously stated, major damage on fabric is considered to be a tear or hole in excess of 16 inches in length. Repairs of this magnitude often require recovering an entire panel or surface. They are considered to be major alterations or repairs and require an A & P mechanic to submit a form 337 for production aircraft. Refer to Advisory Circular 43-13-1B for detailed information. In general, if the tear is greater than 16 inches and the never exceed speed of the airplane is less than 150 MPH, the area is patched as previously outlined except the patch overlap area should be at least 4 inches and the finishing tape used should be 4 inches in width. If the tear exceeds 16 inches and the never exceed speed is greater than 150 MPH the patch overlap should be 4 inches and the finishing tape used over the seam should be 6 inches in width centered on the edge of the patch.
Do not get involved in repairing underlying structure unless you have the ability to do so and, in the case of a production airplane, supervision from a licensed airframe mechanic.
REPAIRS TO FABRIC TOPCOATED WITH AUTOMOTIVE POLYURETHANE
So, how do you repair polyester fabric that has been sprayed with an automotive color coat? First of all, it is not advisable to use automotive polyurethane topcoats over fabric. A number of aircraft are flying today that have used these products as a final topcoat. If you are considering the use of an automotive color coat take my advice and donít use it. A lot of problems will be avoided.
Automotive paints are too brittle to properly adhere to fabric. The topcoat will usually show signs of cracking within 5 years or so. The fabric itself is almost always in good shape but the paint cracks and actually begins to separate from underlying layers. Fabric covering companies and distributors regularly field questions from customers asking what they can do to fix the cracking and separating problem. The short answer is often one people do not want to hearórecover the airplane.
Prior to repairing a damaged area on fabric that was coated with automotive polyurethane, it is imperative that the paint be completely removed. Do not use a paint stripper to remove the paint. Many of the new strippers work good to remove the paint but they will also remove the finishing tapes and weaken fabric cemented seams. In addition, they leave a residue over the fabric that will make recoating a real nightmare. So, what should you do?
You must first remove the polyurethane paint by mechanically peeling it off with a putty knife or a blast of air from an air gun. Sanding is also an option. You can sand down through the paints to the underlying coats then repair the fabric as previously discussed. If you are encountering large areas where the topcoat is peeling and cracking recovering the area entirely will probably be the best fix. It is usually easier to recover than to try to get large areas of problem topcoats off of the fabric. I have seen some airplanes where all chemical coats could be peeled off down to bare fabric. If you can remove all of the chemicals down to bare fabric then you have a chance of salvaging the fabric. If that is the case, clean the fabric thoroughly using MEK and then recoat using the proper chemicals and procedures.
Explicitly follow the recommendations of the fabric covering manual for the process you are using. The Ceconite manual provides instructions for covering an airplane using nitrate and butyrate dope in addition to outlining the procedures discussed for repairing damaged fabric. You can easily accomplish a repair on Ceconite fabric. Just be sure to have all the necessary tools and materials, a copy of the manual, and the supervision or know-how before you begin.