How do you repair minor fibreglass and gelcoat damage on a boat hull?

Clean out the loose material and bevel the edges, then fill shallow gouges with a gelcoat filler or use a thickened marine epoxy for deeper fibreglass damage. Finish by applying a colour-matched gelcoat slightly proud of the surface, sanding progressively from 240-grit up to 1200-grit wet-and-dry, and compounding it to a high gloss.

Assessing and Preparing the Hull

Determine the depth of the damage. Gelcoat forms the coloured outer layer and measures 0.5mm to 1mm thick. If you see woven strands or a yellowish-grey resin layer beneath the white surface, the impact has reached the structural fibreglass laminate. You cannot just slap gelcoat over this. Gelcoat lacks structural strength and will crack.

Wear a P3 respirator mask and safety goggles when grinding fibreglass or sanding epoxy. Use a rotary tool with a cone-shaped grinding stone or a countersink bit in a drill to rout out the gouge. Remove all crushed or flaky material until you reach solid laminate. Bevel the edges of the hole to a 45-degree angle. This increases the surface area and gives the repair material plenty of grip. Wipe the area thoroughly with a clean rag soaked in acetone to remove hull wax, grease and dust.

Filling Deep Fibreglass Gouges

If the impact has breached the laminate, replace the missing structure before dealing with the cosmetics. For a repair the size of a 50p coin, marine epoxy works best.

• Mix a small batch of epoxy, such as West System 105 resin with 205 fast hardener.
• Stir in a high-density filler powder, like West System 406 colloidal silica, until the mixture reaches the consistency of peanut butter.
• Press this paste firmly into the gouge using a plastic spreader to eliminate trapped air pockets.
• Leave the epoxy filling 1mm to 2mm below the surrounding hull surface to allow room for the final gelcoat layer.

Let the epoxy cure fully. This takes 12 to 24 hours depending on the ambient temperature. Once hard, wash the area with water and an abrasive pad to remove the waxy amine blush produced during curing. Rough up the epoxy surface with 80-grit sandpaper so the gelcoat will adhere.

Mixing and Applying Gelcoat

For the cosmetic top layer, use a polyester gelcoat. Off-the-shelf gelcoat fillers work well for older white topsides. For an exact match on newer boats, buy unpigmented gelcoat and a pigment set. Mix tiny amounts of pigment into the gelcoat. Test the colour by dabbing it onto a piece of masking tape stuck next to the repair. Gelcoat darkens slightly as it cures.