Fabric sealer surrounds the fibers in the fabric with a  protective coating to provide adhesion and keep out dirt and  moisture. The sealer is the first coat applied to the polyester  fabric after it is attached to the airframe and heat shrunk to  fit snugly. Dope-based fabric coating systems utilize non- tautening nitrate dope as the primary fabric sealant. The  application of tautening dope may cause the fabric to become  too taut resulting in excess stress on the airframe that could  damage it. Nondope coating systems use proprietary sealers  that are also nontautening.  [Figure 3-9]  remains uncut until an inspection is desired. The rings are  typically plastic or aluminum with an approximately three- inch inside diameter. Spring clip metal panel covers can be  fitted to close the area once the fabric inside the inspection  ring has been cut for access.  [Figure 3-8]  The location  of the inspection rings are specified by the manufacturer.  Additional rings are sometimes added to permit access to  important areas that may not have been fitted originally with  inspection access. The airframe structure of a fabric covered aircraft must be  cleaned, inspected, and prepared before the fabric covering  process begins. The final preparation procedure involves priming the structure with a treatment that works with the  adhesive and first coats of fabric sealant that are to be utilized.  Each STC specifies which primers, or if a wood structure,  which varnishes are suitable. Most often, two-part epoxy  primers are used on metal structure and two-part epoxy  varnishes are used on wood structure. Utilize the primer  specified by the manufacturer’s or STC’s instructions. Modern fabric covering systems utilize special fabric cement  to attach the fabric to the airframe. There are various types of  cement.  [Figure 3-9]  In addition to good adhesion qualities,  flexibility, and long life, fabric cements must be compatible  with the primer and the fabric sealer that are applied before  and after the cement.  Once the aircraft fabric has been installed, sealed, and fill- coat protected, finishing or topcoats are applied to give  the aircraft its final appearance. Colored butyrate dope is  common in dope-based processes, but various polyurethane  topcoats are also available. It is important to use the topcoat  products and procedures specified in the applicable STC to  complete an airworthy fabric re-covering job. The use of various additives is common at different stages  when utilizing the above products. The following is a short  list of additional products that facilitate the proper application  of the fabric coatings. Note again that only products approved  under a particular STC can be used. Substitution of similar  products, even though they perform the same basic function,  is not allowed. After the fabric sealer is applied, a filler is used. It is sprayed  on in a number of cross coats as required by the manufacturer  or the fabric covering process STC. The filler contains  solids or chemicals that are included to block UV light from  reaching the fabric. Proper fill coating is critical because  UV light is the single most destructive element that causes  polyester fabric to deteriorate. Dope-based processes use  butyrate dope fillers while other processes have their own  proprietary formulas. When fillers and sealers are combined,  they are known as fabric primers. Aluminum pastes and  powders, formerly added to butyrate dope to provide the  UV protection, have been replaced by premixed formulas. • A catalyst accelerates a chemical reaction. Catalysts  are specifically designed for each product with which  they are mixed. They are commonly used with epoxies  and polyurethanes.  • A thinner is a solvent or mixture of solvents added  to a product to give it the proper consistency for  application, such as when spraying or brushing. 3-7 Figure 3-8.  Inspection rings and an inspection cover.  Figure 3-7.  Plastic, aluminum, and seaplane grommets are used to  reinforce drain holes in the fabric covering.  Fabric Sealer Fillers Primer Topcoats Fabric Cement