My first car was a 1966 mustang that the original owner had creased almost the whole length along both sides. It has other areas of damage too. Where there was rust, I took off what I could and used a neutralizer. Bondo smoothed it up and then it was painted (I didn't do that). It held together for several years while I was in college. Bondo over rust is not going to work. Bondo over metal, if it isn't too thick, holds together fine. If you watch any of the car shows on TV you can see people putting thin bondo on cars that cost as much as a house. It is not a bad thing but it can be used improperly, like a lot of techniques.