This won't anser all of your question and I know I have been pimping it allot but, look at
zerorust. It is a direct to metal paint that can be top coated.
I have seen posts on a few other boards by guys that have been doing DIY restorations while still driving the rigs have used it as primer and left it for a few months or more before getting a paint job. It's not like regular primer, it seals rust and acts as a moisture barrier so rust can't come back.