My 2 cents on the matter- I used to work for my father in a brit car restoration shop, mostly doing concourse bodywork. I found a couple things out in this process, stripping cars and reworking bodies. Bondo is talc based, meaning that if it isnt properly sealed, it will absorb water and rust out the metal behind it. When you first seal with primer everything is great, but a chip in your paint can end up leading to a hole in the metal behind it. As a result we used a product called "metal 2 metal" from Evercoat. It doesnt change color like bondo does when mixed with hardner, so it has a little bit of a learning curve, but it is aluminum powder based, so it will not absorb water. It also "feathers" on edges better, so its easier to get a smooth finish. Its what I've used so far to cover my badge holders and smooth out my knee dents. Not that I'm done, but it works well...
