Gelcoat and hull care: UV damage and osmosis prevention

Strong sun chalks gelcoat and fades colour. Wash, polish and wax every year, repair chips early, and watch the underwater hull for blisters.

Updated 24 Oct 2026 2 min read

Key points

Wash with mild soap, polish with a fine compound, then protect with wax or a sealant each year.
Fix chips and cracks in the gelcoat before water gets in.
Check the underwater hull for blisters at every haul-out.

What the sun does

Ultraviolet light breaks down the surface of the gelcoat. It turns dull and chalky, and dark colours fade first. In the Med, where the boat sits in strong light for months, the effect is visible within a few seasons if the hull is not protected.

A yearly routine

Wash with fresh water and mild boat soap. Avoid harsh cleaners that strip protection.
Treat stains with a gelcoat stain remover, not a household product.
Compound chalky areas with a fine polishing compound, by hand or with a low-speed polisher. Light passes avoid thinning the gelcoat.
Protect with a marine wax or a polymer sealant. Reapply each year, or twice a year on a boat that sits in the sun.

Repair early

A chip or crack in the gelcoat is a path for water into the laminate. Fill it with matched gelcoat or epoxy filler, sand and polish. A small repair costs little. A neglected one leads to moisture in the laminate and a larger job. See Hull moisture readings and delamination.

Underwater hull

At each haul-out, look for blisters, soft areas and cracks in the antifouling. Osmosis is slow and costly once established, and early treatment helps. A barrier coat of epoxy applied to a dry hull reduces water absorption. See Osmosis: serious or superficial? and The annual maintenance schedule and antifouling.

Drying matters

A hull that spends part of the year ashore dries. See The off-season (invernaje): in the water or on the hard. A hull that stays in the water all year absorbs more, so barrier coats and inspections are worth the effort.

Shade helps

A cover, a bimini or a shaded berth reduces the heat and light on the deck. Fabric covers should breathe, so moisture does not collect underneath.

What to avoid

Abrasive pads and heavy compounds that remove gelcoat.
Pressure washers aimed at seams and fittings.
Silicone polishes before a repair or a repaint, which stop paint from sticking.

Also useful at step 5 Osmosis: serious or superficial?

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