http://www.por15.com/

If you want to replace some rusted metal with fiberglass, you can cut out the metal and then place something in the hole, behind the sheet metal that will act as a 'back wall' of sorts that the fiberglass you lay on top will adhere too. Perforated metal can work. Can use body panel epoxy adhesive to hold it in place. Brush POR15 around the edges where it's adhered to the metal. This would mean you would need some kind of decent access from behind to the panel you are fixing. Once your piece of perf'd metal is in place, lay some fiberglass mat on top and overlap onto the sheetmetal. Probably better to use a fiberglass-reinforced bondo-type product called kittyhair or "long-strand fiberglass body filler". Spread some of that on, smoothly as possible and then sand so it's nice and flush with the surrounding sheet metal. Put a layer of regular bondo as a finishing filler overtop, sand, prime and ready for paint.

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