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Pics of these areas? I can't think of any on mine that would trap dirt and water..

Might help to post which racing organization will sanction your track days. Rules vary, and it's hard to speculate on this without seeing what your Z looks like.

Can you spray foam then fiberglass over the foam. Then sand and paint?

Should be fine as long as you use the fireproof stuff. 240sx guys have been using this method for years, as have I on my track 350z with over fenders. No tech problems to date.