Do you know which product? I have typically used Bondo but I was thinking of going with Evercoat's Rage Gold instead, I've heard its easier to work with. I know everyone has their own favorite filler.
I just talked to John--he said don't use the cheap stuff from the discount auto parts stores. He used USC "duraglas" fiberglass filled filler for the main hole filling. After sanding, he used Evercoat "rage xtreme" lightweight filler. Badd a$$ but pricey stuff he said.
