Primer what to use

epoxy over bare non-rusted clean metal, do your body work on top of the epoxy using 80-150-220 grit paper, then blow on some good high-build primer and block it out using 320 then 500 or 600 (repeat if necessary to remove deeper sanding scratches). then it should be ready for a sealer and top coat of choice.

as gerahead said, use 2K products, in my opinion 1k products (including rattle cans) are not anywhere near as good as a catalyzed/activated material.

I have used some of the summit racing brand primers and in my opinion they are really not bad, not anywhere near as bad as some of the junk they sell in paint stores out here in CA.