Rich38Chevy,
Tiny is right on...etch primer has some ability to halt surface rust on parts that have been blasted and not painted soon after.
This means a very light hint or blush of rust, not stuff you can feel with your hand. The typical etch primer is porous so the item to be painted must be kept indoors and dry.
The best method is to paint with a good etch primer, which is a two part system, then topcoat with another two part (2K) primer which seals the metal completely, and allows you to do the body work right over this 2K primer at your convenience.
You can let the fenders or whatever sit around for eons and the bullet proof primer will protect them. You have probably seen rat rods and the such buzzing around town in primer...outside in the weather. This is 2K primer (probably), so you know it's tough.
I'm not a body and fender man, but this is what was explained to me by the paint store, and that is what I did...and it worked.
Stuart
