well im sorry but i just registered to this forum just to say that is the worst body advice i have ever heard. you shouldnt hand out advice when you dont know what you are talking about.
i have a little bit of a clue however.
1. sand paint off surrounding area
2.cut out the rust if its through the metal or if its surface rust, grind it off.
3. patch the metal with MORE metal not bondo if it is through. then skim coat it with some high quality bondo... if it is not through, skim coat the area with bondo.
4. then sand it, prime it etc..

it is a waste of your time in the long run when you just slab bondo over rust. it will come back to haunt you if you dont do it right.