Question About Sanding

any welding or panel replacement needs to be done before primer. Since you blasted it, weld any holes or replace sections that need to be. Then grind down your welds, wipe down with a pre-cleano or wax/grease remover, and then epoxy prime. Sand the epoxy (so the filler as something to stick into to) and use filler as needed. Then prime with a high build primer ............lots of steps.

You could also use filler over the bare metal, but the epoxy will adhere better to metal than filler will.