How do I apply gelcoat to a boat hull or deck?

Apply gelcoat to fibreglass or sound existing gelcoat after cleaning, drying and thorough abrasion, using the specified catalyst ratio and excluding air during the final cure. Small chips and local patches are manageable DIY jobs, but recoating an entire hull or moulded non-slip deck takes considerable preparation and finishing.

Check the existing gelcoat

Wash the surface, then test a small area with marine cutting compound and polish. Chalkiness, yellow staining and shallow scratches often respond to cleaning and polishing. Unnecessary gelcoat adds hours of sanding.

Gelcoat is usually a pigmented polyester resin, not paint. It restores the surface finish but cannot repair moving cracks, soft laminate or a wet deck core. Investigate cracks around stanchion bases, cleats and deck fittings before covering them.

For a whole topsides repaint, a suitable marine paint system is often easier to finish neatly. Below the waterline, check that the chosen gelcoat and subsequent primer or antifouling system suit continuous immersion. Blistered laminate needs assessment and drying before recoating.

Choose gelcoat and prepare

Buy fresh repair gelcoat matched to the boat, plus its specified MEKP catalyst. Original gelcoat commonly has a shelf life of about six months. Even the original colour may differ from a hull faded by sunlight, so make a cured colour sample before tackling a visible repair.

1. Wash away salt and dirt, rinse thoroughly and allow the surface to dry.
2. Remove wax, silicone and grease with a suitable surface cleaner before sanding. Use fresh cloths to lift contamination away.
3. Remove loose or cracked material. Open small chips into sound edges and feather the surrounding surface; do not simply coat over a crack.
4. Abrade the bonding area, typically with P80 to P120 paper for recoating, following the product instructions. Keep coarse scratches within the area being covered.
5. Vacuum thoroughly, complete the specified final cleaning and mask nearby fittings.

Repair deep damage with a compatible laminate or repair filler first. Polyester gelcoat over epoxy repairs needs explicit compatibility guidance from the suppliers. A hard epoxy surface does not guarantee adhesion, even after sanding.

Wear gloves, eye protection and suitable respiratory protection. Provide ventilation and keep sparks and flames away from resin, catalyst and cleaning solvents.

Mix and apply gelcoat

Work in dry conditions under shelter, typically around 18 to 22°C, with the surface above the dew point. Direct sunshine heats the laminate and can sharply shorten working time.

Many repair gelcoats specify approximately 1 to 2% MEKP by weight, but follow the label for the product and temperature. At 2%, 100g of gelcoat requires 2g of catalyst. Use suitable, accurate measuring equipment. Do not guess by counting drops or add extra catalyst to compensate for cold weather.