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I'd recommend blowing out the dust, etc. behind the panel, maybe even
using brake cleaner and then POR. One person I know went so far as to
fill the bottom of the void with "Great Stuff" expanding urethane foam
to seal it. Interesting idea, but Great Stuff can make a god-awful mess
when it expands if you're not careful. (Might be an interesting idea for
Milano rear wheel arches, however!)
I didn't see a reply to this suggestion on using expanding urethane foam as
a sealer. I had also heard of using this product to fill rockers and other
unibody cavities to stiffen the chassis. Seemed like a sound idea but when
I asked restoration and body specialists, the response was that the foam
creates rust by wicking moisture and staying wet, and accelerated car
cancer from the inside out. Their advice was against using the expanding
foam. Does anyone else have any experience or opinions?