Repairs Cleaning Windshield Installation Installation Procedures Polishing Figure 7-93.  Stop drilling of cracks. 7-57 A A1 A1 A All the strains that originally caused the crack are concentrated at point tending to extend the  crack. Therefore, with a #30 or 1/8" drill bit, drill a small hole at the end of the crack point to  distribute the strain over a wider area. Each crack occurring at any hole or tear is drilled in the same manner. Do not attempt hand polishing or buffing until the surface is  clean. A soft, open-type cotton or flannel buffing wheel is  suggested. Minor scratches may be removed by vigorously  rubbing the affected area by hand, using a soft clean cloth  dampened with a mixture of turpentine and chalk, or by  applying automobile cleanser with a damp cloth. Remove  the cleaner and polish with a soft, dry cloth. Acrylic and  Plastics should not be rubbed with a dry cloth since it is likely  to cause scratches and to build up an electrostatic charge that  attracts dust particles to the surface. If, after removing dirt  and grease, no great amount of scratching is visible, finish  the plastic with a good grade of commercial wax. Apply the  wax in a thin even coat and bring to a high polish by rubbing  lightly with a soft cloth. Whenever possible, replace, rather than repair, extensively  damaged transparent plastic. A carefully patched part is not  the equal of a new section, either optically or structurally.  At the first sign of crack development, drill a small hole  with a #30 or a  1 ⁄ 8 -inch drill at the extreme ends of the  cracks.  [Figure 7-93]  This serves to localize the cracks and  to prevent further splitting by distributing the strain over a  large area. If the cracks are small, stopping them with drilled  holes usually suffices until replacement or more permanent  repairs can be made.  cellulose acetate plastics are thermoplastic. Friction created  by buffing or polishing too long in one spot can generate  sufficient heat to soften the surface. This condition produces  visual distortion and should be avoided. Use material equivalent to that originally used by the  manufacturer of the aircraft for replacement panels. There  are many types of transparent plastics on the market. Their  properties vary greatly, particularly expansion characteristics,  brittleness under low temperatures, resistance to discoloration  when exposed to sunlight, surface checking, etc. Information  on these properties is in MIL-HDBK-17, Plastics for  Flight Vehicles, Part II Transparent Glazing Materials,  available from the Government Printing Office (GPO).  These properties are considered by aircraft manufacturers  in selecting materials to be used in their designs and the use  of substitutes having different characteristics may result in  subsequent difficulties. When installing a replacement panel, use the same mounting  method employed by the manufacturer of the aircraft. While  the actual installation varies from one type of aircraft to  another, consider the following major principles when  installing any replacement panel. 1.  Never force a plastic panel out of shape to make it fit  a frame. If a replacement panel does not fit easily into  the mounting, obtain a new replacement or heat the  whole panel and re-form. When possible, cut and fit  a new panel at ordinary room temperature. 2.  In clamping or bolting plastic panels into their  mountings, do not place the plastic under excessive  compressive stress. It is easy to develop more than  1,000 psi on the plastic by overtorquing a nut and  bolt. Tighten each nut to a firm fit, and then back the  nut off one full turn (until they are snug and can still  be rotated with the fingers). Plastics have many advantages over glass for aircraft use,  but they lack the surface hardness of glass and care must  be exercised while servicing the aircraft to avoid scratching  or otherwise damaging the surface. Clean the plastic by  washing it with plenty of water and mild soap, using a  clean, soft, grit-free cloth, sponge, or bare hands. Do not use  gasoline, alcohol, benzene, acetone, carbon tetrachloride, fire  extinguisher or deicing fluids, lacquer thinners, or window  cleaning sprays. These soften the plastic and cause crazing.