Years ago a buddy of mine went through his '70 chevy pick up, new rockers, cab corners etc. He filled them with foam, I told him I wouldn't because of the extreme temp changes here in Kansas. Warm interior temp and cold outside temp creates moisture. It all rusted up in about 5 years. I don't think this would be an issue with it all being on the interior.
On the underside of the car I usually use a brush on rust inhibitator about a pint will do the whole car underneath but you can use it inside also, it goes on as a white paste and turns black once dried. Then I use a body seam check all around the seams usually found at a paint supply store, after that I will use the standard automotive black spray undercoating found in spray cans. I have had great results with no rust bleed through later on.
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