• Polypropylene is used in many of the bumpers on import cars. The plastic is usually black, and sometimes gray. Polypropylene has a waxy feel to it and it smears and gets ":hairy" when sanded at high speed. If you have determined that the plastic you are going to weld is polypropylene, this is the plastic welding rod you need. Use this plastic welding rod with either Urethane Supply Company's airless plastic welder or a hot air plastic welder. Black color has been added to make more "invisible" repairs to bumpers and other black polypropylene. This package contains 30 ft. (9.1 m) of polypropylene plastic welding rods. Each rod measures 12 inches (30 cm) long and is 1/8th inch (3 mm) in diameter.