How do I choose and apply gelcoat to repair a boat's damaged surface?

For small chips and deep scratches in a fibreglass boat, use colour-matched polyester repair gelcoat with a wax additive or suitable covering film so the exposed surface cures fully. Check first whether the damage stops at the gelcoat or extends into the fibreglass laminate: gelcoat restores the finish but cannot repair weakened structure.

Check the damage

Wash the area, remove wax with a suitable surface cleaner and inspect it under good light. A light scratch that disappears when wet may polish out; a chip exposing sound fibreglass usually needs filling and gelcoat.

Loose fibres, soft laminate, cracks spreading from a fitting or damage that returns after previous repairs need investigation. Repair damaged laminate before applying the cosmetic finish. Blisters below the waterline also need diagnosis before filling.

Check that the surrounding finish is gelcoat. A previously painted hull needs a repair compatible with its paint system; do not spread polyester gelcoat over paint.

Wear nitrile gloves and eye protection. Ventilate when using resin or solvents, keep ignition sources away and use suitable respiratory protection for sanding dust.

Choose gelcoat and match colour

Gelcoat is a pigmented polyester resin, not paint. For a small vertical chip, marine gelcoat repair paste is easier to control than a runny brushing grade. Check that the product suits the repair location, particularly below the waterline.

• Unwaxed gelcoat: stays tacky where exposed to air; use the specified release film or PVA curing barrier on the final surface.
• Waxed gelcoat, often called topcoat or flowcoat: contains an additive that excludes air as it cures, allowing an exposed finishing coat to harden.
• Multiple applications: follow the product's recoating instructions; remove wax and abrade cured waxed material before adding another layer.

Ask the builder or parts supplier for the original colour, quoting the model, build year and hull number. Hallberg-Rassy whites, for example, vary between boats and years; one boat's colour code is not a universal match.

Polish a small area around the damage before matching, because oxidation changes the apparent colour. Even the correct factory shade may differ from a weathered hull. Adjust with tiny quantities of compatible gelcoat pigments before adding catalyst. Make a small cured sample and compare it in daylight.

Buy a small, fresh quantity. Gelcoat commonly has a shelf life of about six months, depending on formulation and storage conditions. Keep it cool according to the label, rather than aboard a hot boat.