20 years ago i used Bondo, it worked, but you could get Ditzler 999 filler in a gallon for less money (see auto paint suppliers) and it was much nicer to work with.

Currently using bondo as i have not much to fix and as SprayTech says, things are better these days.

Bought car from a friend's parents, they paid to have a rusty front door fixed at a reputable shop (rusty and holes). It was bubbling, so i took it apart to fix...it was NEWSPAPER (the sunday comics as a matter of fact) bondo'd in place, bondo'd over, sanded and painted. Even when i cheat, I can do better!! (duct tape, spray foam, roof cement is my favorite!)