Initially, I had put rubber-bottom rugs over the floor. I glued them down with spray adhesive, but they let go within a year and it was wet under them. The rugs provided almost no insulation value.
Left over from building my boat, I had 2 quarts of 2-part expanding polyurethane foam. I taped the holes underneath, mixed and poured on the liquid to a 1/4 depth across the metal floor. 20 minutes later (on a hot day) it expanded to an almost universal thickness of 1.5 to 2 inches. I used a Surform plane after it hardened to smooth/flatten it out, then put 3/8 inch MDO plywood over it and .024 diamond plate aluminum. I pulled the tape. Water and rust were never a problem again.
Polyurethane glue (Gorilla Glue) exposed to air with nothing to keep it from swelling up is single-part expanding polyurethane foam.
Prem
