What is antifouling paint? Put simply, antifouling your boat keeps your hull smooth and clean by preventing the build-up of marine organisms

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The rougher your hull, the more friction it creates and the slower your boat is in the water. It pays to keep a hull smooth, particularly for motorboats where you make significant savings in fuel.

Also, an unwanted side effect of a heavily fouled hull is the translocation of invasive marine species, which is a worldwide concern, especially in shipping.

What are the different types of antifouling paint?

For years, there’s been talk of banning copper-based antifouling, and although laws have tightened up, this hasn’t happened yet.

Copper metal (Cu) and cuprous oxide (CuO) are the ingredients of most metallic biocides found in the best antifouling paints, and how they’re released, depends on the medium they’re suspended in.

Soft antifoul, sometimes called ‘self-polishing’ or ‘self-eroding’, is ideal for yachts such as the PBO project boat Maximus, because it erodes slowly as the vessel moves through the water, so a fresh film of biocide is always on the hull’s surface.

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Similar to soft antifoul is self-polishing copolymer antifouling (SPC). Here, the release of biocides is controlled by how fast the binder dissolves over time, rather than the flow of water over the hull.

With hard antifoulings, the copper is suspended in a solid coating and leaches out slowly. These are suitable for high-speed vessels (over 30 knots) as well as any vessel that dries out.

Racing yachts often use hard antifoulings because it means the hull can be scrubbed clean for better performance without the paint wearing away.

Biocide-free antifoulings are either silicone-based or biocide-free hard coatings. Rather than peeling away (ablating) they make the hull so slippery that it’s difficult for fouling organisms to become attached, or if they do, they’re removed when the boat is in motion.

Each antifouling works in a different way, and the right paint for your boat is determined by your type of vessel, usage, speed, location (fouling levels), cost and colour, as we found out in our 2016 test of antifouling paints.

Why should I use antifouling paint primer?

When you’re starting from scratch with a bare hull – or if you’re applying antifouling on top of an unknown antifoul – it’s important to start with the primer. It’s an essential part of any boat maintenance routine, and serves as a durable and waterproof undercoat, protecting wood, steel, aluminium and fibreglass.

In Maximus’s case, we didn’t know which products had been used previously. She’d been on the hard-standing for two years, and you could flake the antifouling off with your fingers. Had we just painted antifouling over the top we’d have run the risk of the new coat peeling off.

If the hull had been in better condition and we knew which antifouling it was, we could have simply pressure-washed, lightly wet-sanded and applied new antifouling paint on top. But if in doubt, it’s always recommended to re-apply antifouling after a long lay-up.

How does antifouling paint work?

Incredibly, there are over 4,000 different types of marine species that can attach themselves to your boat. These are generally classified as slime, weed and shell fouling.

Maximus was in a boatyard when we acquired her so we hadn’t seen her hull when it came out. However, we did spot the remains of some interesting tube-worm casing on the boat’s propeller.

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There are over 4,000 different types of marine species that can foul a hull. Photo: Chris Pearsall/Alamy

Before we chose our antifouling, we called knowledgeable paint expert Laurie Brebner at Marine & Industrial. “Antifouling is formulated to reduce throughout the season, which is why putting enough on at the beginning of the season is so important.” he said.

“However, the thing to bear in mind is that no matter how expensive the antifoul is, if the boat doesn’t move, your hull will still foul.” Laurie explained that a motion of water across the hull is required for it to work.

“I was speaking at a dealer conference recently, and a lot of people didn’t realise this,” he said. “They were horrified. Boats in marinas, especially in warm climates, will grow a beard if they’re just left.”

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For many boats a pressure wash will be enough to clean up the surface before applying primer or antifoul straight over the old antifouling paint. Photo: Roy Conchie/Alamy

“Essentially antifoul is comprised of three main ingredients: resin which adheres to the hull; a copper-based biocide content that fights against hard shell (tubeworm, barnacles etc); and co-biocides which are chemical formulations that deter the build up of weed and slime.”

The clever part of ablative antifoul is that the resin elutes, exposing equal amounts of biocides, inhibiting sealife growth.

Antifoul coatings work either by hydrolosis, where the resin dissolves slowly and constantly elutes fresh biocide, or by hydration where the resin is washed away by boat movement through the water, allowing the biocide to elute.

With hydration antifoulings the antifoul wears down unevenly and you’re left with a skeleton layer of resin. Laurie suggested to jet-blast this off, and then apply another two coats of the same antifoul. If you do this every season after six or seven years you’ve got a millimetre or two of old stale resin left attached to the hull.

At this point it’ll be time to scrape back to the original hull gelcoat or epoxy and start afresh.


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This feature appeared in the February 2022 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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