1 part epoxy primer and more primer..

I say always bother to read the tech sheet for the product. Never fails for me!
The big deal for epoxy primer is it ability to stick... even on slick cat chit!
On bare metal, I use epoxy primer, any filler on that, I want to get couple of good coats of high filler urethane primer on that epoxy quick too. I call that good, then use the epoxy primer reduced for a sealer, then topcoat.
It takes couple of good coats of epoxy to seal the bare metal for me, and I like the urethane primer on that too, especially if not in a sealed garage!
Disclaimer: Your results may vary!