Satisfied that the bumper is as right as we can make it, we prime the bumper with a special urethane primer that is specially made for rubber front bumpers. Make sure you give the bumper several good coats to allow for sanding. When wet-sanding these primers, be sure to use plenty of water because the primer will tend to ball up. The extra water helps flow out the rolled-up primer. We used 360-wet/dry sandpaper on a semisoft hand-block to block-sand the bumper. Work in an X pattern along the bumper's surfaces.