How to Mold a Rear Bumper in 4 Steps

You may have to replace your rear bumper because you were involved in an accident, or you may choose to replace it with a custom design that includes a spoiler. Either way, you will find the task much easier with a little planning and preparation. You can fabricate your own rear bumper out of steel pipes or a fiberglass mold.

Steel bumpers provide more protection and can be fitted for a trailer hitch. However, they are more costly and difficult to make. Cutting and attaching the pipes requires sophisticated metal working tools.

Fiberglass bumpers are less durable but are suitable for passenger vehicles that are only driven on highways. You can build a mold out of foam or cardboard. Fill the mold with fiberglass sheets and bind them together with resin. The polyester resin produces hazardous fumes so work in a ventilated area.

Step 1 - Remove Old Bumper

The first step is to take off the old or damaged bumper. Use it as a template when designing the new bumper. Note its length, width, thickness, angles, and curves. Decide whether you will replace it with a fiberglass or metal bumper and obtain the necessary materials.

Step 2 - Build Fiberglass Mold

If you are fabricating a fiberglass bumper, build a mold out of foam or cardboard. Hold the bumper between pieces and cut them to shape with the knife. You will have to harden the edges of the mold so it will not bond to the fiberglass later. Apply polyester primer to the mold and allow it to dry. Sand it smooth and polish it gloss. Apply two to three coats of wax.

Step 3 - Fit Pieces to Bumper

If you are using fiberglass, lay the sheets into the mold. Cut and bend them so they can be inserted quickly later. Make six to eight sheets of fiberglass for the entire bumper. For metal bumpers, reposition the trailer hitch and angle frames if necessary. First, cut a length of pipe in half and tack weld the halves onto opposite sides of the hitch. Second, tack weld a long piece slightly above the first, but also close enough to weld to the frame.

Step 4 - Make Bumper

For fiberglass bumpers, spray the mold with a thick application of aerosol gelcoat and let it get tacky. Lay the fiberglass sheets on top of each other, soaking each sheet with resin as you go. Allow the resin to harden before you pry the bumper out of the mold. To remove the bumper without damaging it, try lifting it with a paint mixing stick and compressed air. Attach the new bumper to the vehicle with the original fasteners or equivalent. For metal bumpers, complete the final weld once you are satisfied with the placement of the pipes.