Whatever suits you - most of what you're going to be painting just needs protecting, no one will see it.
If I'm going to be spray painting outside panels I'll use automotive etch primer, with any 2 pack over the top asap, but red oxide and any enamel type would do for you?
If I'm going to be spray painting outside panels I'll use automotive etch primer, with any 2 pack over the top asap, but red oxide and any enamel type would do for you?