I went with a similar approach as jomicheal using spi epoxy as my primer. The one area of concern is that the rust and metal prep products leave a residue in the pores of the metal which only lift out when wiped and rinsed with water WHILE THE PRODUCT IS WET AND ACTIVATED. So just remember that if you let it sit overnight to reapply your product and let it react a bit before washing is off.
Also this would be the place where you practice being anal about getting everything wiped and washed good.
I was nervous as hell when I first started all worried about moisture getting to the metal. I came up with all sorts of ways to do it dry using grinders sanders and drills but in the end in places I couldn't easily remove or prep, I used a marine product called phosphor prime. I just made double sure I got it all off and still took the pressure washer to it followed by the shop vac blower to dry it off.
In places like the inside of the doors when I was redoing the bottoms I knew I wasn't going to be as thorough so I used rustoleum prepped metal primer instead of epoxy eliminating compatibility issues altogether.
The cool thing about epoxy I like is that nothing is stronger or more water tight and that's maybe the one reason I would choose to go epoxy then filler plus as mentioned earlier once you've got it sprayed it can sit and wait for you to get to it.
keep up the good work
