SANDING

determine how many of these operations are necessary. Each step in the process must be performed thoroughly, or subsequent polishing will not remove scratches left by previous operations. Sanding and buffing cause thickness variations in the plastic around the scratch. If skillfully done, these operations will cause only minor optical distortions, which will not be serious in most applications. Distortion may be reduced by gently polishing and feathering a fairly large area around the scratch. In critical optical sections, however, even minor distortions may cause serious deviations in sighting. Such sections, even though scratched, should not be sanded or buffed. If necessary, these sections are replaced. SANDING.—Transparent plastics should never be sanded unless absolutely necessary, and then only when surface scratches, which may impair vision, are too deep for buffing. When sanding is necessary, the finest, smallest grit abrasive paper that will remove the scratch or other defect should be used first. Normally, you will never need abrasive paper coarser than No. 320A; however, abrasive paper as coarse as No. 240A may be used if the situation warrants. The abrasive paper is wrapped around a felt-covered, wooden or rubber block, and the defective area is rubbed lightly, using plain water or water with a 2-percent soap content as a lubricant. Use circular strokes, as shown in figure 14-1. Never use a straight back-and-forth motion. Sand an area about two or three times the length of the defect in order to minimize optical distortion and excessive thinning of the plastic. The initial sanding should then be followed by similar treatments, using successively finer grades of sandpaper in the following sequence: DO NOT SAND UNLESS ABSOLUTELY NECESSARY TO RE- MOVE DEEP SCRATCHES. HOLD SANDPAPER BY SMOOTH RUBBER OR WOODEN BLOCK AND OVER A WIDE AREA TO PREVENT OPTICAL DISTORTIONS. EXCESS PRESSURE IN SANDING OR BUFFING WILL BURN PLASTIC. Figure 14-1.—Proper method of sanding plastic. 14-3