I don't know if it applies, but when I rust proofed the bottom edges of my doors, hood, and tail gate, I first ground them down to bare metal using a wire brush on a drill. I prefer the circular type instead of the cone shaped brushes. Instead of primer, I used Loctite galvinizing compound. My dad gets it from an industrial supply store. It has the same texture as primer. I don't know exactly how well it prevents the rust, but I will know sooner or later. I then brushed a coat of Rust O Leum, and finished with spray cans. It looks good quality wise, and Rust O Lium white matches up to Ford Oxford white pretty well, even in a spot where people don't usually look.
For what it's worth.