Hi charcigar & Tenringr. Sorry for the slow response to your questions. My computer has been down for a few days. I only painted the inside of the rear storage compartment with the POR paint. I bought that POR from the POR people at a auto show & they didn't mention anything about UV rays to me.
Tenringr, I took the rear bumper off to clean as much of the rust out of that storage compartment as I could before painting it. My rear bumper is held on with four large screws. They were tight & hard to get loose to turn. I have a really big screw driver that I hit with a hammer a couple of times to get the screws loose. They came right out after that. Only took maybe ten min to get it off. Went back on easy.
SF CA area