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