Repairing a fiberglass fender involves replacing the damaged or missing bits with new layers of fiberglass and filler, then sanding to the proper shape. The difficulty level of the task depends on how much damage there is. The required materials include fiberglass mat and filler, polyester resin, acetone and a variety of small tools for cutting and shaping. Safety equipment includes a respirator, eye protection and latex gloves.
Start by removing the fender to a well-ventilated work area. Remove the damaged portions with a die grinder. Use acetone on a cloth to remove dust and oils. The fiberglass will need some sort of backing, glued or taped to the inside of the fender, to hold its shape. For small areas, thick plastic sheeting will suffice. Larger areas or complex shapes may require shaped cardboard or wood.
Patches of fiberglass mat are added over the damaged area. Each patch should be a little larger than the one below it and these should be prepared beforehand. Thoroughly mix the polyester resin according to the manufacturer's instructions. The resin can be rolled, brushed or soaked on the fiberglass patches, but drips and air bubbles must be removed before the next layer is added. Once enough fiberglass has been added, allow three hours for the resin to dry.
Once it is completely dry, the repair can be sanded to shape. Seal the repair with a thin layer of fiberglass filler.