Hi Slab, 2 things, first I use a Stainless Wire Cup Brush (not Twisted) on a grinder to buff all the polyurethane foam residual from the hull. Works great. Second White Bead Foam will get heavier than the spray in foam. I have pulled the decks of Aluminum boats that all of them had white bead foam, the foam weighed a ton. Just through condensation the foams can absorb water. Since the underside of your deck was not glassed in the moisture went right thru micro cracks in the deck surface and got trapped in the voids. Time then is all that is needed for the water to penetrate the foam. The expanding foam is still the best material for void flotation. The expanding foam 2# formula also provides some structure to the hull. just my