You will want to use Epoxy primer. I use MetalPrep before I spray the epoxy when I can't sandblast the surface as there are always pits. If this is old metal, sanding will not clean the pits out. Follow instructions carefully as you must clean the surface and neutralize before the epoxy.

Do note that you must paint the epoxy within a week with primer etc, else you will need to scuff the surface when you shoot next.

Astrobuf