taz, Whenever I do a major restrotion on a boat, I drill a hole in the lowest point in the bottom of the hull to let the moisture drain from the foam. It doesn't cause a rust problem in boats because they're made from fiberglass. It is a different story in car or truck. The moisture is held within the porus foam and actually promotes rust over a period of time. I recommend a good rust barrier from behind and a good seam sealer for the joints. Also you can place drain holes in strategic places to allow any moisture from remaining in contact with the rear of metal panels. Good Luck
