How to Make Fiberglass Fender Flares

Updated July 20, 2017

Fender flares extend from the wheel well out to the fenders. Adding bumper flares to your car can keep the mud off the body panels and add resale value to your vehicle. Making a fibreglass bumper flare takes only a few supplies, but it also requires a lot of space, attention to detail and careful planning. Although tedious work, making your own fibreglass bumper flares can save you money compared to purchasing them retail, along with the ability to customise them.

Design the bumper flares on a piece of paper with the measurements specified. Measure the distance from each side of the wheel well at the bottom and from the bottom of the wheel well to the top. From there, connect the measuring points to form a semi circle. You might have to do some tweaking to make the circle fit exactly. Draw out an actual size stencil to match up with the wheel well. Remember that the bottom of the flare must fit flush against the wheel well to properly attach it to the vehicle.

Use a razor blade to carve a foam block per your design specifications. Keep in mind that the mould is the polar opposite of your finished product. Sand the foam with 180-grit sandpaper until the bumper flares are nice and smooth. Cover the foam in Bondo and wait, until dry, before sanding with 220-grit sandpaper.

Spray the polyester primer on top of the Bondo in even coats. Polyester primer is thick, unlike primers for paint. Follow the directions on the can to allow for the proper drying time. After the primer has dried, use 180-grit sandpaper to completely sand down the primer smooth. Continue to sand with finer grits until you are wet-sanding with 1,000-grit sandpaper.

Wax the mould four times over a three- to four-day period. Apply tooling gel to the mould and allow it to get tacky before applying the resin. Separate the sheet of fibreglass into smaller fibres. Lay the fibreglass evenly on top of the resin and use a roller to remove the air bubbles. Repeat this step 10 times to ensure a thick wheel flare. Allow the glass to cure and the resin to dry.

Pry the wheel flare out of the mould with a wooden mixing stick. You can use a knife or a razor blade to remove any protruding pieces of fibreglass.


Always wear gloves to keep the glass and chemicals from sticking to your hands. Also wear a mask to keep from inhaling the fumes.

Things You'll Need

  • Styrofoam
  • Bondo
  • Sandpaper (180- through 1,000-grit)
  • Polyester primer
  • Moulding wax
  • Resin
  • Polyester resin and hardener
  • Tooling gel
  • Plastic mixing cups
  • Brushes
  • Fibreglass roller
  • Fibreglass mat
  • Razor blade
  • Wooden mixing stick
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About the Author

Quyen Tong began writing professionally in 2006 when he launched a nutrition information website named Complex Nutrition. He has graduated from Purdue University with a degree in aeronautical engineering and has submitted technical papers regarding the NextGen project and future flight trajectories for air travel within the United States.