I have used POR-15 on a couple of vehicles, it is super tough stuff, but you do have to topcoat it for UV protection. Unless you are using it on the frame, under the hood, etc where it doesn't get much/any sunlight. I used it on the master cylinder and brake calipers on my Corvette. You can pour brake fluid all over this stuff with no problems. Normal paint would be bubbling and peeling off.
Sherwin Williams has a paint the is made to go right on surface rust. I haven't had the opportunity to try it yet. I have a flat bed trailer with a steel bed that needs spruced up. I'm thinking of trying this paint on the frame, and putting the rhinolinig plastic coating on the bed to help make it a nonslip surface. It's like ice when wet.