Boat Antifouling and Hull Cleaning: When, How and Which Paint

Antifouling, scrubbing and lift-outs: what each job does

Left afloat, the underwater hull collects slime first, then weed, then hard growth such as barnacles. A fouled bottom makes a boat slower and thirstier at the same revs, and costs a sailing yacht some of its ability to point. Antifouling is the paint that slows that growth; hull cleaning removes what has already settled, either afloat or ashore during a lift-out. The two work as a pair: a hull cleaned gently and often keeps its paint longer than one left until only hard scraping will shift the growth.

• Antifouling: a coating that discourages growth. Most release a biocide, and in the UK they are regulated by the Health and Safety Executive (HSE), which must approve a product before it can be sold or used.
• Anode: a block of zinc, aluminium or magnesium that corrodes in place of the propeller, shaft, saildrive leg or other underwater metal.
• Scrubbing piles: posts a boat dries out against at low tide so the hull can be cleaned between lift-outs, where the harbour allows it.
• Wash-down area: a pad on the hard standing that collects, and often filters, the water from pressure washing.

How often should you antifoul and clean the hull?

There is no single interval. Fouling speed depends on water temperature and nutrients, how long the boat sits unused and what paint is on the hull. Haven Quay, a boatyard in Lymington, summed up the usual practice in November 2024: boats that live afloat or are used regularly should be checked every year to decide whether they need repainting, while boats kept ashore and washed down may only need antifouling every other year. Warm water fouls faster than cold, and a boat left on its mooring fouls faster than one in regular use. The best guide is your own hull: note its condition at every lift-out or dive and compare year on year. Signs that it is due:

• At the same revs, speed is down or fuel use is up compared with last season.
• A green or brown beard grows along the waterline and comes back soon after you brush it off.
• A diver finds barnacles on the hull, propeller, shaft or leg.
• The undercoat shows where the water scours hardest: the bow and the leading edges of the keel and rudder.
• The boat has not been out for over a year, so nobody has seen the anodes.

Before the lift-out: a preparation checklist

Part of a good antifouling job happens before the boat leaves the water. Lift-out slots are booked in advance, especially in the weeks before the season, and you want the paint and the anodes on hand on the day.

• Book the lift-out and a space on the hard standing, and ask what the yard includes: pressure wash, chocking or a cradle, moving the supports so you can paint under the pads.
• Find out which antifouling is already on the hull (last year's invoice or record): the new paint must be compatible with it.
• Buy enough paint for the coats you plan, with some spare for the areas that wear fastest.
• Order anodes with the right part numbers and in the right metal for your water: zinc for salt water, aluminium for salt or brackish water, magnesium for fresh water, according to the UK anode maker MG Duff.
• Get the kit together: coveralls, eye protection, a mask, disposable gloves, a ground sheet, masking tape, wet-and-dry paper, scrapers, rollers and trays, plus bags for the waste.
• List the jobs to do while she is out: seacocks and skin fittings, stern gland or shaft seal, cutless bearing, propeller, rudder, transducers.

Antifouling a boat, step by step

How long the job takes depends mostly on drying times and the weather, so leave some slack in the booking. The number of coats, the drying time between them, the minimum application temperature and the launch window come from the paint's data sheet and the tin.

• 1. Lift-out and chocking by the yard. Photograph the bottom straight away, before washing: that is the real state of last season.
• 2. Pressure wash immediately, while the growth is still wet and soft, on a pad that collects the runoff.
• 3. Scrape off barnacles and hard growth without gouging the gelcoat, then rinse.
• 4. Inspect: blisters (a possible sign of osmosis), knocks and cracks, seacocks, skin fittings, propeller, shaft, cutless bearing and rudder. Repairs come before paint.
• 5. Anodes: check every one. MG Duff advises an annual inspection and renewing any anode that is more than 50% wasted; replace a cracked or loose one too. Keep the contact surface clean and never paint an anode, because paint stops it working.
• 6. Preparation: light wet sanding (dry sanding puts biocide-laden dust into the air), degreasing, and primer on any bare patches.
• 7. Mask the anodes, transducers, skin fittings and the boot top.
• 8. Roll on the antifouling: most paints need at least two coats, with the stated drying time between them. The patches under the pads get painted when the yard moves the supports or at relaunch.
• 9. Relaunch within the stated window, then check the seacocks, stern gland and skin fittings for leaks before leaving the berth.

Hard or eroding antifouling: choosing the paint

There is no best antifouling, only the right one for your boat's speed, mooring, water and existing coating. Eroding (self-polishing) antifouling wears away slowly as the boat moves, exposing fresh paint; Practical Boat Owner (October 2023) notes that manufacturers recommend it for cruising yachts. Hard antifouling cures to a tough film that resists wear; the same article says high-speed boats, boats on drying moorings and racing boats need it. Non-biocidal coatings, silicone among them, are another route, and the RYA lists them among the less damaging options. Two compatibility rules matter: eroding paint can go over hard, but hard over eroding should be avoided; and copper-based antifouling must never go directly onto bare metal, while aluminium hulls and drives need a formulation made for them. Five questions before you buy:

• What is on the hull now? If nobody knows, a barrier coat of primer before the new paint removes the compatibility risk.
• Does the boat take the ground, on a drying mooring or against a harbour wall? Abrasion wears an eroding paint quickly.
• Is it fast? On a planing motorboat, an eroding paint wears through where the water scours hardest.
• Is there aluminium below the waterline: hull, outdrive, saildrive leg, propeller? Only products made for it are on the list.
• What does the label say? Users must follow the conditions of use printed on it, as the RYA reminds owners.

Where to do it: wash-down areas, scrubbing piles and waste

Antifouling belongs on hard standing with a way to catch the runoff: a filtered wash-down facility at a marina, yard or harbour, or portable bunding, as the RYA puts it. The Green Blue, the environmental programme run by the RYA and British Marine, adds two habits: keep pressure washing and paint work away from drains, which may lead straight to open water, and wet-sand rather than dry-sand to keep the dust down. If you use scrubbing piles between lift-outs, the RYA's advice is to scrub off the fouling only, not the paint underneath, and to keep old and new paint out of the water. Harbours set their own rules on scrubbing and in-water cleaning, so ask the harbour master before you start. Waste, item by item:

• Washing water: it stays on the pad, which collects and filters it.
• Paint flakes and sanding dust: a ground sheet, wet sanding and an industrial vacuum with a scraper attachment; they go with the hazardous waste.
• Tins, rollers, brushes, trays, rags and gloves: anything contaminated with paint is hazardous waste, for the yard's hazardous waste bins or the local recycling centre.
• Leftover paint: seal the tin and keep it for touch-ups during the season.

Example (fictional): the antifouling record to keep

This record is invented for illustration: a 10 m (34 ft) cruising yacht on a swinging mooring in the Solent, with an inboard diesel and a saildrive. A few lines are enough to buy the right paint and anodes next spring, and to see whether the hull is fouling faster.

• Dates: lifted 18 March 2026, relaunched 21 March 2026, on the yard's wash-down pad; lift and pressure wash by the yard.
• Engine hours at lift-out: 1,412.
• Condition at lift-out (photographed before washing): beard along the waterline, barnacles on the leg and propeller, no blisters.
• Anodes: saildrive leg and propeller anodes renewed, both more than half wasted; part numbers on the invoice.
• Antifouling: eroding, same range as last year, two coats plus a third along the waterline; colour and batch number noted.
• Other jobs: seacocks worked and greased, saildrive seal checked, folding propeller cleaned and greased.
• Watch next season: the leading edge of the keel; plan a full strip when the built-up coats start to flake.
• Who: the yard for the lift and wash, the owner for painting; the yard's invoice attached.

Keeping track of lift-outs across a fleet

With several boats, the hard part is not the antifouling itself but knowing when each hull was done, with which paint and which anodes. In Captain Crews, marine management software, you save the lift-out once as a work template with its checklist, and it can come back every year in the month you choose, one work order per boat. Each step takes a comment, photos are time-stamped, the anodes fitted are recorded when the job is closed and the work order joins the boat's maintenance log. The yard's invoice, read automatically, is filed against the boat, and the owner follows the job from their own account.