machining, or a combination of these with the action of solvents or solvent vapors. Crazing can be caused by improper cleaning, improper installation, improper machining, or cold forming. Once a part has been crazed, neither the optical nor the mechanical defect can be removed permanently; therefore, prevention of crazing is very important. Cleaning Plastic Surfaces 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. NOTE: Water containing dirt and abrasive materials may scratch the plastic surface. A clean, soft cloth, sponge, or chamois may be used to apply the soap and water to the plastic. The cloth, sponge, or chamois should not be used for scrubbing; use the hand method as described for removing dirt or other foreign particles. Dry with a clean, damp chamois, a soft, clean cloth, or a soft tissue by blotting the surface until dry. Rubbing the surface of the plastic will induce (build up) an electrostatic charge that attracts dust particles to the surface. If the surface does become charged, patting or gently blotting with a damp, clean cloth will remove this charge as well as the dust. To clean interior plastic surfaces, dust the surface lightly with a soft cloth. Do not wipe the surface with a dry cloth. Next, wipe carefully with a soft, damp cloth or sponge.   Keep the cloth or sponge free from grit by rinsing it frequently in clean water. Cleaning and polishing compound, Specification P-P-560, may be used to remove grease and oil. Apply the compound with a soft cloth, rub in a circular motion until clean, soft cloth. Removing Scratches From You may be required and polish with another Plastic Surfaces to remove and install 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 14-2