POR15 is good for under the car, as it's out of direct contact with UV light, which breaks it down, unless it's top coated. I've used POR15 before with both good and bad results. But both ways, it's MESSY AS HECK. Rustoleum industrial rust paint is IMO a much better product, because it does the same thing, is not affected by UV light, needs no top coat and is FAR less messy. Much more affordable, too. I got mine for about 27 dollars a gallon at Lowes.
