I was going to do this too as a insulation/sound deadening method until I found out that the foam, once cured, is not very resilient. It tends to break down when the chassis flexes or under vibrations. When it does, the outer coating of the foam is compromised and it does indeed soak up water like a sponge... Excellent rust enhancer.

A product that is used by the off-shore oil industry is called ZRC a zinc-rich paint that actually does provide cold-coated galvanic protection. I plan on using some but, alas, I cannot vouch for it's effectiveness just yet.

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Mike!