[Bicycle Bob]

Thanks, everyone. As far as I know, a speck of rust will always grow under new paint unless it is sealed with epoxy. Everything else is micro-porous due to solvent evaporation or unreacted catalysts. It has certainly stopped my underbody rot. So, I'll probably use a coating of epoxy, but there's still potential for frost damaging the foam where it meets the metal, or building up where it does not. I could drill holes in the foam sheets and do the gap-filling from the middle, and then seal the outside of the foam, but one careless ding and humid air would still get in. So, now I'm thinking of using cotton rope to make drains. It would only admit air slowly, but it would quickly wick away any liquid whenever it thawed.