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See Gordmay''s post of Don Casey''s sealant article above.
"However, the solvents in polysulfide sealant attack some plastics, causing them to harden and split. Specifically, you must not use polysulfide to bed plastic windshields or plastic portlights--either acrylic (Plexiglas) or polycarbonate (Lexan). Don''t use it to bed plastic deck fittings either, including plastic portlight frames. Plastic marine fittings are typically ABS or PVC, and polysulfide will attack both. If you know that the plastic fitting is made of epoxy, nylon, or Delrin, you can safely bed it with polysulfide. Below-the waterline through-hull fittings are in this group, but when there is any doubt, select an alternative sealant . . . For an adhesive seal of plastic components, select a silicone/polyurethane hybrid. An adhesive sealant maintains its seal even when stresses pull or pry the bedded components apart. The sealant stretches like the bellows joining the two sides of an accordion. This accordion effect can be especially useful for plastic portlight installations where the portlights are captured between an inner and outer frame. Although silicone has amazing elasticity, its lack of adhesion means any expansion of the space between the frames is likely to cause the seal to fail."