When I was working on the frame for my 23 I treated it with Ospho to kill any rust that formed from grinding, etc. But before I epoxy primed it I took a DA sander and sanded it all the way off because I was worried about adhesion problems. Supposedly you can take a damp cloth and wipe off the white powder that forms, but I didn't want to chance it.
Ditto, I have used lot of ospho on stuff and never had a problem using epoxy primer over treated areas. I use ospho in multiple stages, cleaning up each one as I go, not leaving any residue or white powder behind. IMO ospho works it's magic in places you can't see or get to, if you can sand it up and make sure your surface is completely clean than it's just that much better.
i know in the oilfield they en-rust and come back in 24 hrs and paint over it,id think youd at least san it smooth before you painted a car-truck,still i wouldnt want to leave anything behind i hate stripping the paint especially fresh paint