cheap primer for a spray gun

Well I’m certainly not an auto body and paint guy, hopefully someone familiar will chime in, but I am familiar with priming very large quantities of steel. And most normal off the shelf primers need to be top coated soon after the primer goes down. If it’s down to bare metal, I would use an epoxy primer to seal it off. That’ll protect the metal for a while and give you some time to plan your attack for paint.