Figure 3-35.  A patch over this small hole on a polyurethane top coat is repaired in accordance with the repair instructions in the STC  under which the aircraft was re-covered. It requires only a two-inch fabric overlap and scuffing into the top coat before cementing and  refinishing. Other STC repair instructions may not allow this repair.  Large section panel repairs use the same proprietary adhesives  and techniques and are only found in the instructions for  the process used to install the fabric covering. A common  technique for replacing any large damaged area is to replace  all of the fabric between two adjacent structural members  (e.g., two ribs, two longerons, between the forward and rear  spars). Note that this is a major repair and carries with it the  requirement to file an FAA Form 337. or cotton fabric maintenance and repair procedures. Refer to  AC 43.13-1, Chapter 2, Fabric Covering, which thoroughly  addresses these issues.  You may encounter a cotton fabric-covered aircraft. In  addition to other airworthiness criterion, the condition of the  fabric under the finished surface is paramount as the cotton  can deteriorate even while the aircraft is stored in a hangar.  Inspection, in accordance with the manufacturer maintenance  manual or AC 43.13-1, should be diligent. If the cotton  covering is found to be airworthy, repairs to the fabric can  be made under those specifications. This includes sewn-in  and doped-in patches, as well as sewn-in and doped-in panel  repairs. Due to the very limited number of airworthy aircraft  that may still be covered with cotton, this handbook does  not cover specific information on re-covering with cotton References to fiberglass surfaces in aircraft covering STCs,  AC 43.13-1, and other maintenance literature address  techniques for finishing and maintaining this kind of surface.  However, this is typically limited to fiberglass radomes and  fiberglass reinforced plywood surfaces and parts that are  still in service. Use of dope-based processes on fiberglass  is well established. Repair and apply coatings and finishes  on fiberglass in accordance with manufacturer data, STC  instructions, or AC 43.13-1 acceptable practices. Mildew,  moisture, chemicals, or acids have no effect on glass fabric  when used as a structure material. For more information on  glass fabric, refer to AC 43.13-1(as revised). Cotton-Covered Aircraft Fiberglass Coverings 3-24