My old 40 Cadillac that I had they used brass! It looked like the guy was a plumber and just used what he had on hand. This was on the pontoon front fenders where the entire outer fender had rusted clean away
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Shot at 2009-10-09
Its hard to see in the photo but there was also about four inches of Bondo on top of the other atrocities they commited.
The spray foam would actually work really well for filling in the larger holes because you can make a pattern for the outer panel with duct tape. When the foam hardens you remove the tape, them file the foam to shape. You can them lay a thick layer of fiberglass cloth and resin on top of that. Its amazing the lengths I will go to to avoid buying a welder LOL