Surface Preparation for Touchup In the case of a repair and touchup, once the aircraft paint  coating has been identified, the surface preparation follows  some basic rules. The first rule, as with the start of any paint project, is to wash  and wipe down the area with a degreaser and silicone wax  remover before starting to sand or abrade the area. If a whole panel or section within a seam line can be  refinished during a touchup, it eliminates having to match  and blend the topcoat to an existing finish. The area of repair  should be stripped to a seam line and the finish completely  redone from wash primer to the topcoat, as applicable. The  paint along the edge of the stripped area should be hand- sanded wet and feathered with a 320-grade paper.  Mix the selected topcoat paint that is compatible for the repair.  Apply two light coats over the sanded repair area, slightly  extending the second coat beyond the first. Allow time for  the first coat to flash before applying the second coat. Then,  thin the topcoat by one-third to one-half with a compatible  reducer and apply one more coat, extending beyond the first  two coats. Allow to dry according to the material data sheet  before buffing and polishing the blended area.  If the damage did not penetrate the primer, and only the  topcoat is needed for the finish, complete the same steps that  would follow a primer coat.  Paint touchup procedures generally are the same for almost  any repair. The end result, however, is affected by numerous  variables, which include the preparation, compatibility of the  finishing materials, color match, selection of reducers and/or  retarders based on temperature, and experience and expertise  of the painter. For a spot repair that requires blending of the coating, an area  about three times the area of the actual repair will need to  be prepared for blending of the paint. If the damaged area is  through the primer to the substrate, the repair area should be  abraded with 320 aluminum oxide paper on a double-action  (D/A) air sander. Then, the repair and the surrounding area  should be wet sanded using the air sander fitted with 1500  wet paper. The area should then be wiped with a tack cloth  prior to spraying. The most experienced painter, the best finishing equipment,  and newest coatings, do not produce the desired finish on  an aircraft if the surface was not properly prepared prior  to refinishing. Surface preparation for painting of an entire  aircraft typically starts with the removal of the paint. This  is done not only for the weight reduction that is gained by  stripping the many gallons of topcoats and primers, but for the  opportunity to inspect and repair corrosion or other defects  uncovered by the removal of the paint.  Apply a crosscoat of epoxy primer to the bare metal area,  following the material data sheet for drying and recoat  times. Abrade the primer area lightly with 1500 wet or dry,  and then abrade the unsanded area around the repair with  cutting compound. Clean and wipe the area with a degreasing  solvent, such as isopropyl alcohol, and then a tack cloth. Before any chemical stripping can be performed, all areas  of the aircraft not being stripped must be protected. The  Stripping the Finish Figure 8-22.  Chart for solvent testing of coating. 8-20 Methanol  S IS IS IS PS IS PS IS IS IS IS IS S IS S ISW IS IS S S S S ISW S ISW IS IS IS IS IS IS IS S IS IS IS SS VS S VS ISW S ISW ISW ISW Butyrate  dope Poly-tone Poly-brush Poly-spray Synthetic enamel Acrylic enamel Nitrate dope Nitro-  cellulose  lacquer Acrylic lacquer Urethane enamel Epoxy paint Hitrate Toluol (Toluene) MEK (Methyl ethyl ketone) Isopropanol Methylene chloride IS – Insoluble  S – Soluble ISW – Insoluble, film wrinkles SS – Slightly Soluble PS – Penetrate film, slight softening without wrinkling  VS – Very Soluble 3–5 Minute Contact With Cotton Wad Saturated With Test Solvent