Since you guys started with Bondo as the topic: I started using fiberglass (kitty hair, short strands) first on any dent area. Theory is that this is waterproof, and then very thin coat of bondo on top. To keep the bondo thin so there is no cracking in the future. Of course, my goal is to beat out as much of any dents as possible prior to adding any filler. By the way, I usually get my projects media blasted to bare metal first, often have the blaster primer the whole car. I sand down with 40-80 grit to get bare metal for the fiberglass. So far this method has worked for years for me. But I am no expert.