How do I apply gelcoat in layers when repairing a boat?
I've chipped the white gelcoat on the cockpit side of my fibreglass boat, leaving a shallow patch about 40 mm across. Should I fill it in one go or build up several layers, and how long should I leave between coats?
Answer
Apply repair gelcoat in two or three layers, letting each gel before adding the next, and leave the finished patch slightly proud for sanding. Use unwaxed gelcoat between layers, then exclude air from the final surface with an approved wax additive, PVA curing film or suitable release film so it cures hard enough to finish.
Assess and prepare the repair
Check whether the damage stops at the gelcoat or reaches the fibreglass laminate. Exposed but intact laminate can accept a cosmetic repair; broken fibres, softness or recurring cracks need the underlying damage repaired before gelcoat goes on.
Gelcoat is a pigmented polyester resin. Use it as a surface finish rather than a deep structural filler. For a deep gouge, rebuild the substrate with a compatible repair system first, leaving room for the gelcoat finish. Applying polyester gelcoat over an epoxy repair requires specific compatibility and preparation instructions.
- Wash away salt and dirt, dry thoroughly, then remove wax and grease with a suitable cleaner before sanding.
- Remove loose edges and sand the repair with 80-grit abrasive, keeping the scratches inside the area you will fill.
- Sand the immediate surrounding margin with 180 to 240 grit. Keep this margin small: coarse scratches outside the patch take considerable work to remove.
- Remove dust and clean as the gelcoat supplier specifies. Allow any cleaning solvent to evaporate fully.
Provide ventilation and wear chemical-resistant gloves and eye protection when mixing. Use suitable respiratory protection for vapours and sanding dust, and keep MEKP catalyst away from skin, eyes and ignition sources.
Match colour and mix batches
Use fresh gelcoat matched to the existing finish. Original factory gelcoat can differ from an older hull because sunlight changes the colour. Make a small cured test sample before tackling a conspicuous repair. Stored gelcoat commonly has a shelf life of about six months, depending on the product and storage conditions.
Work in dry conditions, out of direct sunlight. A practical working range is 15 to 25°C; some boat instructions permit up to 30°C, but warmer material gels faster.
The usual hardener range in boat repair instructions is 2 to 3% MEKP catalyst, but follow the gelcoat manufacturer's instructions for the amount and whether to measure it by weight or volume. Where specified by weight, 100 g of gelcoat needs 2 to 3 g of catalyst. Measure accurately and mix thoroughly, scraping the sides and bottom of the cup.