Fiberglass is a strong and durable material but it is brittle compared to metal. Instead of denting when impacted, fiberglass will fracture or break if hit hard enough. However, when fiberglass cracks or breaks it can easily be repaired and restored to it's original strength. Fiberglass is applied in layers of fiberglass mat that's saturated with a catalyzed fiberglass resin. Small fiberglass repairs such as a door, that don't require a lot of strength, can be done with a fiberglass filler putty. All fiberglass materials can be found at your local auto parts supplier or a marine supply store.
Things You'll Need
Put on safety glasses and a respirator and grind the damage smooth using a dye grinder. Grind away any broken or damaged fiberglass until only solid, intact fiberglass is left. Gradually taper any edges on the damage.
Clean the area with a clean rag and acetone, removing any dust or other residue that may be on the repair surface.
Put on rubber gloves and mix the fiberglass filler with the catalyst in a small bucket, following the recommendations on the containers. Use a stir stick to thoroughly work the catalyst into the filler. When the putty is one consistent color it's ready to apply. After the catalyst is mixed in you will have 15 to 20 minutes before the filler completely hardens.
Apply the filler to the damage using a putty knife. Smooth the filler out with the putty knife so it's level with the surrounding fiberglass. Let the filler harden.
Put on the respirator and sand the filler down with a palm sander and 200 grit sandpaper. Sand the filler until it's flat and smooth and completely blends in with the surrounding fiberglass.
In order for the fiberglass to harden quickly the air temperature should be 70 degrees or warmer.
Always wear a respirator when working with fiberglass.