Fixing A Crack In Your Fiberglass Fender

If you are in the process of restoring an older vehicle, you will most likely need to dabble in the art of fiberglass repair. Fiberglass is durable and light, and it is used on many vehicles for this reason. When you get a crack or tear in the fiberglass of your vehicle's fender, it will be a little messy to repair, but the end result will be worth the headache. Here are some instructions on how to repair a fiberglass fender.

Materials You Will Need:

  • A die grinder
  • A rag
  • Acetone
  • Thick plastic sheeting
  • Spray-on glue
  • Fiberglass mat
  • Polyester resin
  • Catalyst
  • 200 grit sandpaper
  • Foam roller


You will first need to remove the fender from your vehicle to make it easier to get to the tear and to save the rest of your vehicle from the chance of getting chemicals on it that should not be there. Find an area to work where it is well ventilated. Use a rag to rub down the area with acetone, and allow it to dry. Remove the cracked area completely using a die grinder. Once the fender has a uniform hole in it, from the inside of the fender, attach a piece of thick plastic sheeting with some spray-on auto glue. This will give the fiberglass some reinforcement and something to adhere to when hardening.

Using The Fiberglass Mat

Fiberglass comes in the form of a mat which can be torn into smaller pieces. You will need several layers of the fiberglass to build it up to the desired thickness to match the existing body. Each layer needs to be cut a little bigger than the previous until you have a stack of fiberglass totaling the thickness desired. When stacked, the fiberglass will taper from smallest to largest. This is done so it is not stacked in one clump. The tapering will adhere better and will in the end be stronger than if the pieces were the same size.

Applying To The Fender

Mix the polyester resin with the catalyst according to the package directions. You can mix it in a bowl and stir it with a paint stirrer. Dip your foam roller inside and roll a coating over the area that is being repaired. Take the smallest piece of fiberglass and saturate it in the resin mixture. Center it over the area being filled and place it on the resin you just applied, smoothing into place. Continue with each piece of fiberglass, taking a larger piece each time. When all pieces have been applied, allow it to dry for at least three hours.

Finishing It Off

Use 200 grit sandpaper to sand the entire area, making it completely smooth to the touch. Use a rag moistened with acetone and clean the area completely. When dry, mix the fiberglass filler with catalyst according to instructions. It will make a putty that you can apply to the area. Sand down once again when hardened.