Depends on how bad they are. On my daughters Xterra, it was almost white on the gray bumpers when we bought it and nothing worked except flexible bumper gray paint. Scrub them down with water and Dawn soap and a bristled brush, let it dry, tape, and shoot. The stuff goes down, covers easily, dries fast, and is very durable. After done, keep your favorite UV protection dressing on it and it will look nice forever. If not too bad, might try a gel tire dressing like the one from Armor All. It works well on tires and made my daughters look a lot better, but still had light gray streaks so I had to repaint. If it is plastic and not rubber, Meguiars makes a Show Glaze wax that may work for you too. If you havent ever tried it on your paint, you are missing out. The stuff is amazing and really shines and protects. Good luck!