I've got a question. Do you guys do anything to the little areas underneath the car that can collect water easily? Like the valley next to the pinch welds around the whole side, or inside the holes if the frame-rails. I was thinking about Great Stuff expanding foam, but I don't know if that would seal water out, or help trap it in there.

what's the problem?

I'd like to know this, too. Before I paint under there, should I seal any open areas along the floorbard/pinch welds. There is no seam sealer in there now, just nasty undercoating.

there never is any seam sealer under the car, just in the wheels wells.

Undercoating works.

dont use spray foam.