Nah - there aren't any stainless steel panels other than any you would happen to whip up yourself.

I can't imagine this foam doing anything other than making the rust worse. You really cant stop moisture from getting inside - at least on a structure as casually assembled and flexible as a car body. And the foam would make sure it stayed in for as long as possible once it got there. How would the foam 'neutralize' water that got in? Soak it up? Chemically convert it?

You might be better off suirting some thick oil in them every couple of years. I think the Brits have something called 'Waxoyl' for that.

Undercoating on an old car gradually goes from being a rust preventer (soaks up a rock chip without causing a paint chip) into a rust encourager (loss of adhesion makes little pokects that hold water inside against the steel.
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