Figure 3-34.  Applying a primer with UV blocking by spraying  cross coats. the finish of another process. STCs are issued on the basis  of the holder having successfully proven the effectiveness of  both the materials and the techniques involved.  When the fill coats have been applied, the final appearance  of the fabric covering job is crafted with the application  of various topcoats. Due to the chemical nature of the fill  coating upon which topcoats are sprayed, only specified  materials can be used for top coating to ensure compatibility.  Colored butyrate dope and polyurethane paint finishes are  most common. They are sprayed on according to instructions.  Once the topcoats are dry, the trim (N numbers, stripes,  etc.) can be added. Strict observation of drying times and  instructions for buffing and waxing are critical to the quality  of the final finish. Also, note that STC instructions may  include insight on finishing the nonfabric portions of the  airframe to best match the fabric covering finish.  Repairs to aircraft fabric coverings are inevitable. Always  inspect a damaged area to ensure the damage is confined  to the fabric and does not involve the structure below. A  technician who needs to make a fabric repair must first  identify which approved data was used to install the covering  that needs to be repaired. Consult the logbook where an  entry and reference to manufacturer data, an STC, or a field  approval possibly utilizing practices from AC 43.13-1 should  be recorded. The source of approved data for the covering  job is the same source of approved data used for a repair.  Polyester Fabric Repairs  Applicable Instructions Repair Considerations This section discusses general information concerning repairs  to polyester fabric. Thorough instructions for repairs made to  cotton covered aircraft can be found in AC 43.13-1. It is the  responsibility of the holder of an STC to provide maintenance  instructions for the STC alteration in addition to materials  specifications required to do the job.  The type of repair performed depends on the extent of the  damage and the process under which the fabric was installed.  The size of the damaged area is often a reference for whether  a patch is sufficient to do the repair or whether a new panel  should be installed. Repair size may also dictate the amount of  fabric-to-fabric overlap required when patching and whether  finishing tapes are required over the patch. Many STC repair  procedures do not require finishing tapes. Some repairs in  AC 43.13-1 require the use of tape up to six inches wide. While many cotton fabric repairs involve sewing, nearly  all repairs of polyester fabric are made without sewing. It  is possible to apply the sewing repair techniques outlined  in AC 43.13-1 to polyester fabric, but they were developed  primarily for cotton and linen fabrics. STC instructions for  repairs to polyester fabric are for cemented repairs which  most technicians prefer as they are generally considered  easier than sewn repairs. There is no compromise to the  strength of the fabric with either method. Patching or replacing a section of the covering requires  prepping the fabric area around the damage where new  fabric is to be attached. Procedures vary widely. Dope-based  covering systems tend toward stripping off all coatings to  cement raw fabric to raw fabric when patching or seaming  in a new panel. From this point, the coatings are reapplied  and finished as in the original covering process. Some  polyurethane-based coating processes require only a scuffing  of the topcoat with sandpaper before adhering small patches  that are then refinished.  [Figure 3-35]  Still, other processes  may remove the topcoats and cement a patch into the sealer  or UV blocking coating. In some repair processes, preshrunk  fabric is used and in others, the fabric is shrunk after it is in  place. Varying techniques and temperatures for shrinking  and gluing the fabric into a repair also exist.  These deviations in procedures underscore the critical nature  of identifying and strictly adhering to the correct instructions  from the approved data for the fabric covering in need of  repair. A patch or panel replacement technique for one  covering system could easily create an unairworthy repair  if used on fabric installed with a different covering process.  3-23