Bmusick

This is my first post here, I have been looking around the site and have learned a lot. I have experience on the mechanical side (have been a technician for 10+ years.) I have no body work experience, but I am going to learn as I go. I purchased a 1973 240z today and can't wait to start working on it. The car has very little rust, the only area that needs much work is the roof. It has a small crack/chip with rust underneath. I pealed back the bondo/body filler and there is a large spot of surface rust. My question is do I peal all body filler off the entire roof? Or peal of until there is no more surface rust and then sand and re-fill with body filler? Thanks for any help you can give me. Here is a picture, it does not look as bad in person. T