A plastic car bumper can be a bit tricky to paint compared to chrome-coated metal. The first step is to thoroughly clean the bumper with soap and water. Then you will need to scuff the surface with a fine-grade sandpaper block. Once you have scuffed the surface, you will then need to apply at least one coat of primer, though two coats is better. Use a primer that is specifically designed to be applied to plastic. Once the primer has dried, apply a minimum of two coats of auto paint to the surface. Once that has dried, you can then apply a clear protective layer to the bumper.