- 1). Create a mold for your fiberglass bumper. Using the stock bumper as a guide, cut foam blocks into the bumper's shape using a carving knife or a heat-based foam cutter, then tape the blocks together if needed with duct tape.
- 2). Coat the mold's surface with a polyester primer (sanding the primer can help even it out, but watch out for damaging the tape), then apply two to three coats of mold-making wax. As you wait for each wax coat to dry, cut your sheets of fiberglass into smaller pieces that will be easier to lay on the mold.
- 3). Apply three coats of gelcoat spray to the mold's surface. As you wait for the gelcoat to get tacky, mix up the polyester resin. The resin hardens quickly, so be prepared to start applying it soon.
- 4). Use a large paintbrush to apply a coat of the resin to the mold's surface. While this coat is wet, lay the fiberglass sheets on top of the mold so they can stick to and absorb the resin. Remove any air bubbles from the surface with either the brush or a roller.
- 5). Continue applying resin and fiberglass sheets about four to six times, laying one final resin coat over the last sheet. Once the last resin coat hardens, remove the fiberglass from the mold by gently prying it off with a flat tool like a trim or mixing stick.
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