Masking paper should be left on the plastic as long as possible. When it is necessary to remove the masking paper from the plastic during fabrication or installation, the surface should be remasked as soon as possible. Either replace the original paper or apply masking tape. If the masking paper adhesive deteriorates, making removal of paper difficult, moisten the paper with aliphatic naphtha, Federal Specification TT-N-95, type II. Plastic so treated should be washed immediately with clear water. For exterior surfaces, flush with plenty of water, and use your bare hand to gently feel and dislodge any dirt, sand, or mud. Then, wash the plastic with a wetting agent, Specification MIL-D-16791, and clean water.

You may be required to remove and install and polish with another canopies, escape hatches, and other aircraft structures that contain plastic sections. The finish of the plastic must be protected. Plastic is very soft as compared to other aircraft structural materials. The surface is easily scratched or damaged, and should be protected by the use of proper protective covers and storage racks, which are provided by the aircraft manufacturer or manufactured locally. It is easier to avoid scratches than to remove them. It is possible, however, to restore even a badly scratched surface to a good finish by buffing and sometimes sanding. Aircraft MIMs and SRMs specify limits on the length, width, and depth of cracks, and in what areas they are allowed. These measurements are normally made by the use of an optical micrometer. If a scratch exceeds the specified limitations, the surface must be replaced.

Before you sand or buff, be sure the plastic surface is clean. The buffing wheels and compounds should also be free of dirt and grit to avoid seriously scratching the surface during the polishing operation. If the buffing wheels have been used before, remove any hardened tallow by running the wheels against a metal edge.

It is important to remember that most plastic enclosures are thermoplastic and soften when heated. The friction of sanding or buffing too long or too vigorously in one spot can generate enough heat to soften or burn the surface. Also, plastic that has been deep-drawn, or formed to compound curvatures, has a tendency to return to its original thickness when excessive heat is applied. The best procedure is to keep either the wheel or plastic constantly in motion relative to one another. Keep the pressure against the wheel to a minimum, and change the direction of buffing often.

The procedures for removing scratches are as follows: A single deep scratch or imperfection is reduced by sanding to a number of small, shallow scratches. These scratches, in turn, are reduced to a larger number of still smaller scratches on a buffing wheel to which a fine abrasive is applied. These finest scratches are further reduced or filled in with tallow or wax. A final buffing or polishing brings the surface to a high gloss. The depth of the scratch will 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.

Nos. 400A, 500A, and 600A. Wash the plastic after each operation. During each step, the deeper scratches left by the preceding grade of abrasive should be removed.