Though fiberglass is a lightweight and durable material, it can still get damaged. Fortunately, repairing a fiberglass fender is not an overly complicated process since it uses a secondary bond which is basically a repair patch that can be as strong as the original when applied correctly.
This process replaces the damaged fiberglass with new layers that fit seamlessly with the surrounding layers. Though the job is often messy, it is easy to get used to and if the repairs are carried out correctly, they should last as long as the car itself.
Step 1 - Prepare the Damaged Surface
Before you begin, remove the fender from the car and place it on a protected level surface. Use the die grinder to remove the damaged area of the surface thoroughly. Wipe the surface down using a rag which has been dampened in acetone. When this is completely dry, you can continue. Cover the back of the repaired area from the inside of the fender. Use thick plastic sheeting for this and attach it firmly in place using a spray-on glue. This will help to make sure that the fiberglass will be supported until the repairs are complete.
Step 2 - Prepare the Fiberglass Mat
Cut off sections of the fiberglass mat so that they are the same size as the area that need to be repaired. You will likely need multiple layers of this. You will need to make sure you have enough layers to bring the repaired area back up to the thickness of the original fiberglass. Each layer should be a little large than the previous one so that the repaired area will not form a ledge and will instead taper gradually.
Step 3 - Mix the Polyester Resin
The polyester resin should be mixed with about 2 percent catalyst as directed by the instructions on the packaging. Use a paint stirrer to thoroughly mix it. Apply a coat to the repair area using a felt roller. Make sure that any bubbles of air and drips are properly eliminated. Apply the sections of fiberglass mat to the repair area, starting with the smallest one first. Saturate it in the polyester resin mix and continue in the same manner with the following layers until they have all been placed. Allow three hours for the fiberglass to harden.
Step 4 - Sand the Repaired Area
Sand down the repaired area using a 200 grit piece of sandpaper. Using a rag soaked in acetone again, wipe down the area.
Step 5 - Apply the Fiberglass Filler
Mix the fiberglass filler with 2 percent catalyst and make it into a putty by working the catalyst into the filler. Apply this over the repaired area and allow it to harden. Finally, sand it down so that the repaired area blends seamlessly in with the surrounding fender.