I've not done these things but if you go with spray foam, get the low expansion foam. It's the cans rated for door and window frame filler. If you're filling a compartment, first line the space with an open plastic bag, then spray foam into the bag. That way the foam won't bond to the hull/bulkheads. -----Original Message----- From: Klaas <santaklaas@telus.net> To: Montgomery boats groups <montgomery_boats@mailman.xmission.com> Sent: Mon, Feb 27, 2012 7:29 pm Subject: M_Boats: Buoyancy I'm still working on my 15ft Monty and noticed that the old styrofoam is reaking up . That's in both the front and back areas , at the back there was ust two small blocks about 8x10 inch by 1ft . I'm thinking of either filling he areas with some of the building foam in aerosol cans , or using sailing inghy buoyancy bags . Anyone done either or have any ideas why one shouldn't do ither hanks in advance for any comments Laas