The problem with the por-15 is the existing factory wax coat. It would be imposable to scrub it out and I don't think the por-15 would stick. It would also be hard to use the por-15 surface preparation products in there.

What about spray expandible foam? 'Great Stuff'
Wouldn't that stuff fill every cavity? Isn't it water-proof?

Yes, but it is a open cell foam, and could possibly trap moisture in there.

That expandable foam attracts and holds water worse than Bondo. Ive seen it used in a few cars, and when water gets in there it just sits, and the body part rusts out from the inside.

I stopped and looked at an RV last year and noticed that the factory had used that expandable foam alot underneath the rig and then they use undercoating to seal its fate! Talk about a rust sandwich! Then the owner came out, I paid blah blah blah for this 12 years ago. Whatever, see ya. What a POS.