A fiberglass garage door is light but strong. The surface is also easy to clean but the door itself may break if you hit it with a car, or tractor. Repairing the doors requires a little work and depending on how large the crack is could be extensive. For very big cracks, your best bet is to replace the entire fiberglass panel with a new one direct from your manufacturer. For smaller cracks, a quick and easy fiberglass repair is all you need.
Clean off the damaged area with soap and water and a rag or sponge. Remove all dirt and grease. Dry the area completely with a towel.
Sand away all burrs or raised edges on the crack in the fiberglass with sand paper. Cut away larger burrs with a razor blade if necessary. Sand the crack to a smooth edge. If the damage is a hole, bevel sand the edge of the hole with sand paper and cut away the loose shards with a razor blade, if there are any.
Cut fiberglass sheets to fit over the hole or damage as needed. Small cracks with no gap will not need fiberglass sheets but large holes wider than an inch will.
Mix up resin and hardener in a container and apply it to the sheets. Place the sheets over the area of damage and allow the resin to harden completely. This may take up to 24 hours depending on your climate and the amount of hardener used in your resin. Or, for small holes simply apply the resin compound over the damaged area and let it harden.
Sand down the repair to make it flush with the rest of the door using 200 grit sandpaper. Wipe away the sanding dust with a towel. Apply fiberglass primer to the repair. Let this dry and apply the top coat to the repair area. If necessary, repaint the entire panel of the garage door with matching top coat, exterior paint.