sheets should be stacked on the larger ones to avoid unsupported  overhead.  Storage  of  transparent  plastic sheets presents no special fire hazard, as they are slow burning. Masking paper should be left on the plastic sheet as long as possible. You should take care to avoid scratches and gouges, which may be caused by sliding sheets against one another or across rough or dirty tables. Formed sections should be stored so that they are amply supported and there is no tendency for them to lose their shape. Vertical nesting should be avoided. Protect formed parts from temperatures higher than 120°F.  Protection  from  scratches  may  be  provided  by applying  a  protective  coating  of  masking  paper  or  other approved  materials. If masking paper adhesive deteriorates through long or improper storage, making removal of paper difficult, moisten  the  paper  with  aliphatic  naphtha,  which  will loosen the adhesive. Sheets so treated should be washed immediately  with  clear  water. CAUTION Aliphatic naphtha is highly volatile and flammable. You should exercise extreme care when using this solvent. Do  not  use  gasoline,  alcohol,  kerosene,  xylene, ketones,  lacquer  thinners,  aromatic  hydrocarbons, ethers, glass cleaning compounds, or other unapproved solvents  on  transparent  acrylic  plastics  to  remove masking paper or other foreign material, as these will soften  and/or  craze  the  plastic  surface. NOTE: Just as woods split and metals crack in areas  of  high,  localized  stress,  plastic  materials develop,  under  similar  conditions,  small surfaces fissures called “crazing.” These tiny cracks are approximately perpendicular to the surface, very narrow in width, and usually not over 0.01 inch in depth. These tiny fissures are not only an optical defect, but also a mechanical defect, inasmuch as there is a separation or parting of the material. Once a part has been crazed,  neither  the  optical  nor  mechanical defect can be removed permanently; therefore, prevention of crazing is a necessity. When it is necessary to remove masking paper from the plastic sheet during fabrication, the surface should be remasked as soon as possible. Either replace the original  paper  on  relatively  flat  parts  or  apply  a protective coating on curved parts. REINFORCED PLASTICS Glass  fiber  reinforced  plastic  and  honeycomb  are used in the construction of radomes, wing tips, stabilizer tips, antenna covers, fairings, access covers, etc. It has excellent  dielectric  characteristics,  making  it  ideal  for use in radomes. Its high strength/weight ratio, resistance to  mildew  and  rot,  and  ease  of  fabrication  make  it equally suited for other parts of the aircraft. The manufacture of reinforced plastic laminates involves the usc of liquid resins reinforced with a filler material.  The  resin,  when  properly  treated  with  certain agents known as catalysts, or hardeners, changes to an infusible  solid. The reinforcement materials are impregnated with the resin while the latter is still in the liquid (uncured) state. Layers or plies of cloth are stacked up and heated under pressure in a mold to produce the finished, cured shape.  Another  technique,  culled  “filament  winding,” consists  of  winding  a  continuous  glass  filament  or  tape, impregnated with uncured resin, over a rotating male form. Cure is accomplished in a manner similar to the woven cloth reinforced laminates. Glass  fiber  reinforced  honeycomb  consists  of  a relatively thick, central layer called the “core” and two outer  laminates  called  “facings.”  (See  figure  1-22.) The  core  material  commonly  used  in  radome construction consists of a honeycomb structure made of glass cloth impregnated either with a polyester or epoxy or  a  combination  of  nylon  and  phenolic  resin.  The material is normally fabricated in blocks that are later cut on a band saw to slices of the exact thickness desired, or it may be originally fabricated to the proper thickness. The facings are made up of several layers of glass cloth, impregnated and bonded together with resin. Each layer of cloth is placed in position and impregnated with resin before another layer is added. Thicker cloths are normally used for the body of the facings, with one or more layers of liner weave cloth on the surface. The resins are thick, syrupy liquids of the so-called contact-pressure type (requiring little or no pressure during  cure),  sometimes  referred  to  as  contact  resins. They  are  usually  thermosetting  polyester  or  epoxy resins.  Cure  can  be  affected  by  adding  a  catalyst  and heating,  or  they  can  be  cured  at  different  temperatures by adjusting the amount and type of catalysts. Inspection 1-43