Covering With Dope (Nitrate And Butyrate) By Ron Alexander

This article will focus on covering or recovering a fabric airplane using Ceconite fabric with nitrate and butyrate dopes. I will outline each step involved in covering your airplane using this system.

To begin our discussion, if you are covering a production aircraft, you have a restriction requiring you to apply a fabric covering process that meets or exceeds the specifications of the original fabric process that was installed by the manufacturer. To do this you must use a covering system that has been issued a Supplemental Type Certificate (STC) by the FAA. In addition, the materials that comprise this STC must be manufactured according to a Parts Manufacturing Approval (PMA) also issued by the FAA. Your particular type of airplane must be listed under the STC of the respective fabric process for you to use it as a replacement for the original fabric. Ceconite maintains a listing of all aircraft that are approved to use their respective system.

Each approved fabric process must have a manual outlining how to apply the fabric and coatings. This manual is an integral part of the STC and must be adhered to during the covering procedure. To use different fabrics or chemicals that are not part of the process voids the STC. What does this mean? If you are going to mix one type of fabric with another type of tape that is not part of the manufacturer's STC your airplane is not legal. If you mix and match different chemicals in the same way the covering system is not legal. The best advice is to choose a covering system and follow their manual explicitly. They have tested their products for years and know what works and what will cause problems.

If you are covering an experimental aircraft you are free to choose any type of fabric and chemicals you desire to complete this process. However, common sense should prevail. If you do not follow the manufacturer's covering manual you will have the same disastrous results that certainly will occur from mixing systems. Your experimental aircraft flies through the same airspace as a production airplane and from a maintenance standpoint it should be treated with the same care. Just because a custom aircraft builder can legally mix systems and experiment they should avoid any temptation to do so. Otherwise, I can almost guarantee that you will be recovering your airplane within a short period of time.

Ceconite maintains an STC that allows you to use Ceconite polyester fabric along with certain types of chemicals. One specific type of chemical that is approved is nitrate and butyrate dope. However, the nitrate and butyrate dope you use must possess a PMA (Parts Manufacturing Approval). You must ensure the nitrate and butyrate dope has been manufactured with a PMA. How do you know the product is legal? If the product is approved for use on Ceconite fabric under their STC it will state on each product container the words "FAA-PMA approved." So look for that on each can. To use any other type of dope will void the STC on a production airplane. This is a recent change to the Ceconite STC (September, 2000) so be sure you are using the proper products. The Ceconite STC number for all airplanes is SA4503NM. This number should be noted on the FAA form 337 that will be completed by a licensed mechanic after your airplane has been recovered.

Lets get down to the business of covering. Last month we noted that you may recover a production airplane without a mechanic's license if you work under the supervision of a licensed airframe mechanic. Doing you own fabric work will save you literally thousands of dollars. Typically, a shop will charge $8,000+ to cover your airplane depending upon the type and size. This along with approximately $2,500 for materials adds up to a lot of money. If you follow the Ceconite manual you can install fabric and coatings on your airplane that will not only have a quality appearance but that will provide a service life of over 15 years. So lets get to work and learn the steps of fabric covering.

Tools Needed for Covering

Other than the spraying equipment, the tools required for fabric covering are not expensive. Several of them you probably already have around the house or shop. An ideal list of fabric covering tools follows:

  • sturdy sawhorses about 3 feet high covered with carpet scraps
  • snag free table for cutting fabric
  • electric clothing iron ( 1100 watts minimum)
  • small hobby iron
  • thermometers and heat sink for calibrating the irons
  • an effective respirator
  • outside breathing source if using polyurethane paint for final topcoat
  • inch glue brushes
  • paint brushes
  • rib lacing needles
  • sharp scissors
  • good quality pinking shears
  • single edge razor blades
  • chalk snap line
  • measuring tape
  • paint stirring paddles and straining cones
  • soup ladle for dipping paint
  • spring clamps with 2 inch throats to clamp fabric
  • wood spring clothes pins to hold fabric
  • T-head pins
  • tack cloths
  • cotton rags
  • paint cans for use in applying chemicals
A word of caution concerning fabric covering tools. I do not recommend the use of a heat gun. If you have one-hide it during this stage. It is impossible to calibrate the heat emitted by a heat gun. Also, the temperature changes as the gun's distance from the fabric changes. Your fabric covering manual will explain the importance of proper temperature when shrinking polyester fabric. A calibrated iron must be used - not a heat gun. I will discuss spraying equipment when that step is presented.

Steps of Covering Using Dopes

  • Removal of old fabric (restorations)
  • Preparation of surfaces
  • Inspection of surfaces
  • Selection of fabric type
  • Attachment of fabric to structure
  • Shrinking fabric
  • Seal the fabric surface using nitrate dope
  • Secure the fabric to the wings
  • Apply inspection rings and drain grommets
  • Apply finishing tapes
  • Apply butyrate fill coats
  • Protect the fabric from UV rays of the sun
  • Apply final color coats of butyrate dope

    We will examine each specific step in this and subsequent articles.

    Removal of Old Fabric

    Often a builder or aircraft restorer will have the mistaken idea that fabric covering begins when you cement the fabric in place. This is far from reality. As a matter of fact, a lot of time and effort will be needed prior to ever cutting the fabric for placement on the aircraft. Anyone who has ever restored an airplane certainly knows that most of the total work involved is in the preparation phase. A few basics need to be presented concerning adequate preparation. First of all, if you are recovering your airplane, take care in removing the old fabric. This is very important. You can save yourself a lot of time and effort by carefully cutting the old fabric away leaving the inspection plates, drain grommets, reinforcement patches, and control cable cutouts in tact. This will help you with placement of these items on the new fabric. Taking pictures or videos of the airplane prior to removing the fabric may also be helpful. You should be careful not to damage underlying structure. Any tendency to rip and tear off the fabric must be avoided. Take your time and map everything out so it will go back together more easily. Obviously, if you are building an airplane this step will be omitted.

    Preparing Surfaces for Covering

    Preparation of each surface may vary somewhat depending upon the material involved. Usually, you will be encountering aluminum, steel, wood, or fiberglass as the underlying structure.

    Whether you are preparing a steel, aluminum, or wood structure, do not use any of the familiar one-part zinc chromate primers or any of the spar varnishes typically found in hardware stores. The dopes applied to the fabric will soak through and may lift these chemicals causing rust of metal surfaces and rotting of wooden surfaces.

    You should use only two-part epoxy primers and varnishes on metal and wood surfaces. The dope will not affect them. Before priming or varnishing a surface be sure it is completely clean and free of all oil and other contaminants. Do not prime over rusted pieces. Remove the rust and immediately prime. A piece of bare steel will rust within hours if a primer is not in place. Make sure you fill all dents in leading edges, etc. I would recommend a product called SuperFil rather than Bondo. Bondo is polyester filler that will shrink with age. I do not recommend using Bondo on aircraft surfaces unless you are prepared to redo the filled portion of the surface after it shrinks and cracks the topcoat. SuperFil is epoxy filler. That means it will not shrink over time. It may be used on wood, fiberglass, or metal with equally good results.

    What if you have old flaking varnish on a wooden surface? You do not need to remove all of the old varnish. Simply dry sand the varnish scale removing the loose parts. After sanding, wipe the surface with a paint cleaning solvent to remove grease, oil, or other contaminants. Wipe dry with a clean cloth and varnish with an epoxy varnish.

    Old aluminum must have any corrosion removed using Scotch Brite pads or aluminum wool. Never use steel wool or a steel brush on aluminum. Old aluminum must then be acid etched, treated with a conversion coating, and then primed with an epoxy primer. If you are covering over new aluminum it should be treated with a conversion coating and then primed.

    Often you will encounter dented leading edges on wings. There are a couple of ways to fix these dents. If they are very pronounced replacement of the aluminum may be necessary. If the dents are not too deep you may want to use a cloth padding to cover the area. Polyester cloth padding is often used between the leading edge aluminum or plywood and the fabric itself.

    All of the sharp edges that could potentially cut the fabric should be covered with anti-chafe tape. This usually involves rivet heads, metal seams, and sharp edges. Let your sense of touch be your guide. If you feel something sharp cover it with the anti-chafe tape. Do not use masking tape for this purpose. It will retain moisture and cause problems later on. Also, paper masking tape will turn brown with age and possibly show through a light colored paint.

    After preparing all surfaces for cover, you should have each part inspected for damage and proper preparation. This is a very important step. If you are working under the supervision of a licensed mechanic, that person should inspect each component part prior to your attaching the fabric. If you are a licensed mechanic it is advisable to have another person look over your shoulder to find problem areas you may have missed. Look for damage, proper safetying of hardware, sharp edges, etc. Often you will find tools that are left inside a wing or fuselage. Be sure to remove any tools, hardware, etc. that may have been left by mistake.