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
