Aquarium grade silicone adhesive is neutral on the foam, being that grade it does not release the acids in curing that others do, it will stall your rehaul of the liner but not stop it. Some more people on the site may have their favorites, others I have read detest silicone...There is a grade made for special fiberglass applications and it must be ordered, most people use what is in the hardware store and are disappointed being generally disastrous... its not common knowledge there is a difference in the product applications beyond what the general hardware people carry. I would state the case that even in the day the choices in materials were behind the curve to save costs and the life of the fiberglass trailer woefully underestimated, these guys as you know have many lives... The best job I ever saw done on this type of issue was to re-ensolited with a current product and then add a plastic bed liner material sprayed over it, no seams, muss or fuss and it looked awesome...but that is for its re-birth maybe.
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There are several spots on my camper where the foam with the white stuff has sagged (top corners ) is there a spray glue that I can inject under the insulation and push it back up? Contact cement or other solvent based cements would certainly eat the foam, has anybody tried sticks - a - foam, this is used on that blue styrofoam.