POR-15 does not react well to UV rays. It will discolor the paint. I have not seen it with the black but I have with the silver.(I have both) It does not affect the toughness of the paint. It turned the silver a little yellow. If you have a area that is going to see direct sunlight-top coat it. If not, like the underside of the car, I would not be concerned. It is good stuff[img]/phpBB/images/smiles/icon_tup.gif[/img][img]/phpBB/images/smiles/icon_tup.gif[/img]