Not only does por 15 make a mess of future repairs, if you don't have perfect surface prep and adhesion, it can accelerate rust. This is because it is an impermeable urethane that will hold any water that gets between it and the steel. It will maintain its integrity with itself,effectively hiding the failure from view.
Based on my research, I would use an epoxy primer or zinc phosphate /chromate as John suggests, followed with cavity wax (applied after welding).