Paint Removal: Are we there yet, Daddy?

[SuperDave]

OK, I admit I don't know what I'm doing. Be gentle. The project will be a race car, so it doesn't need to be concourse restoration guality. Of course, I want it to look good and last a long time.

Stripping and sanding off the paint, some parts of the car are clean and now look great under a coat of primer. But there are some large sections of bondo and numerous smaller sections of bondo on the roof. There are also greenish spots that look like rust that has been repaired. Below are some pictures.

My question is: How much farther do I have to go? Obviously, if I see unstable brown rust, I want to nip that. Do I need to dig out all the bondo down to the bare metal and re-do it completely? Do I dig it out just a little and add some new filler to smooth it out, or do I just want to take the paint off the top of the bondo and smooth everything out as best I can and smooth it out more with the hi-build primer? When do I know I'm finished stripping and can prime?