Fiberglass is a good material to create vanity car parts like designer bumpers. The sheets can be molded into the shape you want and you can then create that shape by mixing with a polyester resin. In order to properly mold a fiberglass bumper, you must first create a mold that will help you shape the fiberglass sheets into the size and shape you need. You also must make sure the fiberglass itself won't bond to the mold.
Create the mold--also called the plug--for your custom bumper, using the original bumper for a guide with whatever materials work for you. You can carve blocks of foam into the shape you want, or you can place cardboard around the old bumper. Make sure the plug will fit the car, then add whatever customizations you want, like spoilers or holes for lights.
Apply a polyester primer to the plug's surface, then sand down the primer with 180 grit sandpaper until the primer is even. Polish the surface to a glossy finish and then let it sit for a day or two. Wax the bumper with a special mold-making wax, wait for it to dry and wipe away excess wax with a clean cloth; repeat the waxing 2 to 3 times.
Prepare the mold for the fiberglass by applying an aerosol gelcoat to the surface; the thicker the gelcoat, the better. Apply three coats to the surface and wait for it to get tacky. While you wait, mix the polyester resin as its instructions tell you.
Apply one coat of the resin to the bumper's surface with a brush, then lay the fiberglass sheets onto the surface while the resin is wet. It can help to cut the sheets into smaller pieces beforehand so they will lay onto the surface easier without large creases or other uneven surface problems. Smooth away any air holes with the brush or a roller.
Add more layers of fiberglass by applying another coat of resin, making sure the previous fiberglass sheet is soaked, and lay the next sheet on top. Lay 3 to 6 fiberglass sheets on the mold this way. Apply one final coat of resin to the last sheet and wait for the resin to dry and harden.
Pry the finished bumper off of its mold/plug. Use a flat, strong object like a mixing stick to help gently pry it off. Blowing air in between the bumper and mold can also help.
