Originally Posted by psneeld
What I am getting at is... are you using home canned stuff or a marine 2 part that really isn't supposed to absorb water (in a realstic timeframe)?
the cheaper home stuff in a can is like a sponge no matter it says on the can...I've tried it...maybe some is truly "closed cell" but I haven't found any...unless the "pond and stone" stuff is any better, The surface is water resistant but break that and the inside is spongy when it comes to absorbing water.
If you want water"proof" foam...what about 2 part marine stuff?
The problem arises over time. Uncoated foam, including the 2 part stuff, will oxidize opening cells and absorbing water, especially if the skim coat is nicked. Because the stuff gives off cyanide gas when it burns it should be coated with a fire resistant paint to address that safety concern, but also to inhibit oxidation. As noted, if you want optimal protection, highly reduced epoxy or epoxy paint sprayed on first, followed by fire-resistant paint would seem to be recommended.
Lots of foam is used for insulation and condensation control, but like everything in boats, it represents a compromise. Easy to apply, kills you if it burns. That's why a number of builders won't use it, preferring USCG approved fiberglass batts so condensation can run to the bilges rather than collecting locally in a broken down area of foam. I tried to get the best of both worlds on Delfin by applying waterproof acoustical cork to the steel first, followed by fiberglass batts. The result seems to be no condensation, no fire hazard and a quiet full. On a steel circa 1960's Vripak I looked at (Constance in Anacortes), the yard glued balsa to the hull, then coated that with a thick layer of tar. That worked for condensation and noise control, but I should think the tar would burn like doxy if given the opportunity.