exceeds the specified limitations, the surface must be
aliphatic naphtha, Federal Specification TT-N-95, type
replaced.
II. Plastic so treated should be washed immediately
with clear water.
Before you sand or buff, be sure the plastic surface
is clean. The buffing wheels and compounds should
For exterior surfaces, flush with plenty of water,
also be free of dirt and grit to avoid seriously scratching
and use your bare hand to gently feel and dislodge any
the surface during the polishing operation. If the
dirt, sand, or mud. Then, wash the plastic with a wetting
buffing wheels have been used before, remove any
agent, Specification MIL-D-16791, and clean water.
hardened tallow by running the wheels against a metal
NOTE: Water containing dirt and abrasive
edge.
materials may scratch the plastic surface.
It is important to remember that most plastic
A clean, soft cloth, sponge, or chamois may be used
enclosures are thermoplastic and soften when heated.
to apply the soap and water to the plastic. The cloth,
The friction of sanding or buffing too long or too
sponge, or chamois should not be used for scrubbing;
vigorously in one spot can generate enough heat to
use the hand method as described for removing dirt or
soften or burn the surface. Also, plastic that has been
other foreign particles.
deep-drawn, or formed to compound curvatures, has a
Dry with a clean, damp chamois, a soft, clean cloth,
tendency to return to its original thickness when
or a soft tissue by blotting the surface until dry. Rubbing
excessive heat is applied. The best procedure is to keep
the surface of the plastic will induce (build up) an
either the wheel or plastic constantly in motion relative
electrostatic charge that attracts dust particles to the
to one another. Keep the pressure against the wheel to a
surface. If the surface does become charged, patting or
minimum, and change the direction of buffing often.
gently blotting with a damp, clean cloth will remove
The procedures for removing scratches are as
this charge as well as the dust.
follows: A single deep scratch or imperfection is
To clean interior plastic surfaces, dust the surface
reduced by sanding to a number of small, shallow
lightly with a soft cloth. Do not wipe the surface with a
scratches. These scratches, in turn, are reduced to a
dry cloth. Next, wipe carefully with a soft, damp cloth
larger number of still smaller scratches on a buffing
or sponge. Keep the cloth or sponge free from grit by
wheel to which a fine abrasive is applied. These finest
rinsing it frequently in clean water.
scratches are further reduced or filled in with tallow or
wax. A final buffing or polishing brings the surface to a
Cleaning and polishing compound, Specification
high gloss. The depth of the scratch will determine how
P-P-560, may be used to remove grease and oil. Apply
many of these operations are necessary. Each step in the
the compound with a soft cloth, rub in a circular motion
process must be performed thoroughly, or subsequent
until clean, and polish with another soft cloth.
polishing will not remove scratches left by previous
operations.
Removing Scratches From Plastic Surfaces
Sanding and buffing cause thickness variations in
You may be required to remove and install
the plastic around the scratch. If skillfully done, these
canopies, escape hatches, and other aircraft structures
operations will cause only minor optical distortions,
that contain plastic sections. The finish of the plastic
which will not be serious in most applications.
must be protected. Plastic is very soft as compared to
Distortion may be reduced by gently polishing and
other aircraft structural materials. The surface is easily
feathering a fairly large area around the scratch. In
scratched or damaged, and should be protected by the
critical optical sections, however, even minor
use of proper protective covers and storage racks,
distortions may cause serious deviations in sighting.
which are provided by the aircraft manufacturer or
Such sections, even though scratched, should not be
manufactured locally. It is easier to avoid scratches than
sanded or buffed. If necessary, these sections are
to remove them. It is possible, however, to restore even
replaced.
a badly scratched surface to a good finish by buffing
SANDING.--Transparent plastics should never be
and sometimes sanding.
sanded unless absolutely necessary, and then only when
Aircraft MIMs and SRMs specify limits on the
surface scratches, which may impair vision, are too
length, width, and depth of cracks, and in what areas
deep for buffing. When sanding is necessary, the finest,
they are allowed. These measurements are normally
smallest grit abrasive paper that will remove the scratch
made by the use of an optical micrometer. If a scratch
or other defect should be used first.
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