A Lost Art
Fabric covering restoration
By Katie Heuermann
Working on fabric covered airplanes is almost a lost art. Many of us hone our fabric skills in A&P school, only to never put those skills to use after we get our certificate. However, there are still many fabric aircraft flying, and many more stored with clipped wings — waiting to take to the skies again. With the cost of all aircraft rising, restoring an old classic may be the only way for someone to realize the dream of flying. Tackling a restoration project may require a bit of review of what was learned in A&P school.
In the old days, the covering options were Grade A cotton or Irish linen fabrics, with nitrate, and later, butyrate dope. But, increases in cost and the short lifespan of these natural fabrics helped drive the market towards other materials. Today, there are many options. Some processes use a fiberglass fabric, some use polyester, others use vinyl instead of dope.
The original processes had a lot working against them. The natural fabrics tended to break down quickly from sun and mildew, and required covering every few years, especially since most aircraft were not hangared as they are now. The polyester fabrics are used now and though impervious to battery acid, still require protection from UV rays. If the UV blocking coats are not used, the fabric will lose 70 to 85 percent of its strength in one year if left outside. However, with a properly applied coating, the fabric’s service life is practically infinite.
More important than the longevity of the fabric covering is the flammability. The original cellulose dope was nitrate. It was extremely flammable, and butyrate was soon substituted. The flammability factor went down some, but it was still flammable, even after years of being on the aircraft. Poly-Fiber® Aircraft Coatings System and Covering System advertises that it is not flammable after the solvents have evaporated out after application. Poly-Fiber will not support combustion. The fabric itself is combustible, but after the process has been applied per the STC and cured, it will not burn — an important consideration when choosing what process to use. Ray Stits, developer of this system and founding member of EAA’s Chapter 1, demonstrates the flammability characteristics on samples of different processes in his video "Fabric Covering with Ray Stits."
Starting with a good foundation is essential. Ensure all structure is free of defects.
The first step in any restoration project is to start with a good foundation, in this case the framework. The steel tubing should be inspected to see if it has good welds, is corrosion free, solid, and is in reasonable alignment. It should be cleaned, prepped, and primed as necessary. Any bad pieces must be cut out and replaced using the welding method the maintenance manual or AC 43.13.1B recommends. Steel is normally used for overall strength, with wood or aluminum used to give form or shape, as in wing leading edges, longerons, ribs, and bows. Ensure any wood is free of cracks and rot, and is intact. Some glue joints may need help. Aluminum should be checked for proper alignment, cracking, corrosion, or any other damage.
Each process has its own products to be used in a given sequence, and for the most part they are not interchangeable. Poly-Fiber has its own STC, as does Ceconite®, etc. If the aircraft is covered in Poly-Fiber, and only one wing needs to be recovered, Poly-Fiber has to be used to cover it. The same process must be used on the entire aircraft. If recovering an entire aircraft that previously had Ceconite, and you want to go with Poly-Fiber now, first make sure Poly-Fiber has an STC for that aircraft, then file a Form 337 for a major alteration when the job is done. If your aircraft is not listed on the STC’s approved aircraft list, plan on a field approval.
Buy the manual for your chosen process, which will have a copy of the STC in it. This manual will take precedence over AC 43.13, since STC’s are approved FAA data. In order for the STC to be signed off legally, the STC manual must be followed to the letter. Some have videos available, and I would recommend getting that, too. Of course, the manufacturer’s maintenance manual is required. Having AC 43.13 is a good idea for any questions not covered in the STC manual or the manufacturer’s maintenance or service manual. Just remember which publication takes precedence, and prioritize which will be consulted first, second, and third.
Securing the fabric
These instructions are general guidelines. Consult the appropriate manuals for specific instructions applicable to your project. Some STC’s will have you buy fabric by the yard, while others have a choice of by the yard or a pre-sewn sleeve for areas like the wings. For the fuselage, the fabric is glued to the tubing structure and to itself for the overlap seam using the special glue called for in the STC Procedure Manual. The amount of overlap in specific areas will be defined in each STC manual. Some aircraft will have the fabric clipped, sewn, or screwed to metal strips in certain areas to keep it aligned. The manufacturer’s maintenance manual will have the specifics on this. Some wings will need to be rib stitched. If neither the STC manual or maintenance manuals explain how to do this, AC 43.13 still has instructions on how to do the different stitches. Essentially, the STC manual will pertain to the fabric process; the manufacturers manual will define what specifics for their particular aircraft; and AC 43.13 will have general guidelines and instructions on how to do a baseball stitch or a rib stitch if neither of the other manuals calls out for it.
Iron it smooth
Once the fabric is glued, most of the modern processes call for lots of ironing with a temperature-calibrated iron. The fabric is ironed at a low setting first, then at a higher setting to get the desired shrinkage and shape around the form. If the iron is not heat- controlled and gets too hot, it could loosen the fibers in the fabric permanently, rendering the fabric unworkable. Take it off and start again. There is no way to fix this problem. Ironing is time consuming to get it done right, but if not done correctly, the appearance of the finished product will suffer. This step, especially, is where having good technical assistance can be invaluable — there’s no substitute for experience in this step.
Next, the fabric gets its first sealing coat. This is usually brushed on. If you are doing Ceconite, or a dope-based process, the butyrate will stick to nitrate, but it will not stick to the polyester fabric. The highly flammable nitrate must go onto the raw fabric. Then, butyrate goes on over that. Poly-Fiber is a vinyl product that does not use dope in its process. The first coats in that system will be Poly-Brush to the raw fabric.
Using a temperature-calibrated iron to obtain desired shrinkage and shape.
From here, the rib lacing is done, as is any other attachment needed for the fuselage and tailfeathers by use of screws, clips, stitching, or whatever is specified in the aircraft manufacturer’s maintenance manual. Then, the reinforcing tapes are applied over the stitches or attaching hardware. These tapes are also used on seams, leading and trailing edges, and fuselage longerons and stringers where two layers of fabric are needed for reinforcement. This tape is made of the same material as the covering fabric. Tapes come in different widths, and usually have pinked edges that give the tapes over 40 percent more edge area. At the same time the tapes are applied, the rings for the future inspection holes and their fabric coverings need to be put on (to be cut later), as well as the drain grommets. If using a dope process, remember these tapes are raw material, and nitrate dope must be used to apply them. Poly-Brush™ is the Poly-Fiber brand name for the product that is used until the aluminum pigmented Poly-Spray coats are applied. After more ironing to smooth tapes and any fabric that may have been missed before, the first spray coats go on. Then, the sanding starts. Spray, sand, spray, sand. Iron some more. Then go in with your UV blocking aluminum pigment coats. Sand some more, and iron if needed.
Color can go on if you are satisfied with the product at this point. With Ceconite, the color can be colored butyrate, which will have a flat, matte finish. It will not adhere to metal surfaces or fiberglass. For these areas, enamel can be matched to the butyrate color. Urethane can be used on fabric, metal, and fiberglass. It will have a wet-look finish. Poly-Fiber colors are called Poly-Tone™ for the matte finish color, and Aerothane™ is the brand name for their urethane product that gives the wet-look finish. Poly-Tone is not good for metal or fiberglass.
One thing to keep in mind when deciding which way to go: In either system, the matte finish products are easy to blend if a repair is ever needed. The urethane wet-look colors will be impossible to blend after repairs. A typical spray gun and compressor system is adequate, but an HVLP spray system is best for all spray coats, especially for the color coats. A respirator needs to be worn anytime spraying is going on — nothing less than a fresh air source respirator when spraying urethanes.
Damage with either of these methods can be repaired. The size of the damage and the area it is in will determine the type of repair. Consult AC 43.13 and the STC manual for the best way to proceed.
Rejuvenating the finish
The covering job should last many years if applied and cared for properly. Yet over time, the finish may, especially with dope products, need to be rejuvenated. Ringworm (fabric defect) is not too common anymore if the correct product was used originally, but for cracking and brittleness, rejuvenators are available. These products soften aged, dry paint, and they add plasticizers that give flexibility to the finish. These can be used only on the matte finish colors. Urethane colors cannot be rejuvenated. After the rejuvenator is used, new topcoat paint goes on again. It is still work, but not as much as a total recover.
After the recover or repair job is complete, remember that the job isn’t finished until the paperwork is done. Just a log entry is required for small repairs. A logbook entry and a Form 337 will be required on total recover jobs. The Procedure Manual is part of the STC, and should be referenced. If only one wing is covered, it still requires a 337 as a major repair under FAR43 Appendix A. That section is clear about what is and is not a major repair or alteration. If there is any doubt, check the regs. The aircraft will also need to be weighed after any major recovering and amended weight and balance entered into aircraft records.
Your pride will soar when you see an old bird take to the skies again — one that was recovered through the nearly lost art of fabric restoration.
Photos used in this article were taken at American Champion Aircraft manufacturing facility in Rochester, Wisconsin. Although they use a slightly different approved process for the Ceconite fabric installation, including the use of pressure sensitive tape, the photos were useful in illustrating the various steps in fabric restoration.
About the Author
Katie Heuermann is an A&P/I.A. and Private Pilot. She is the owner of Penrose, Colorado-based Chinook Air.