How does boat bottom paint work?
I need to repaint the bottom of my 34-foot cruiser this spring. The yard offers 'hard' and 'ablative' options, but I don't really understand how antifouling paint actually keeps the hull clean or which type I should be paying for.
Answer
Boat bottom paint, or antifouling, works by continuously releasing a controlled amount of biocide, usually cuprous oxide, into the water next to the hull to create a toxic boundary layer that marine organisms cannot survive in. Depending on the type of paint, it achieves this either by slowly wearing away to expose fresh copper or by letting water penetrate a hard, porous resin to dissolve the biocide out.
The Biocide Boundary Layer
The active ingredient in most traditional antifouling is copper. When you buy a 2.5-litre tin of paint for £90 to £140, you are paying for the cuprous oxide content. This typically ranges from 25% to 65% by weight. As the paint sits in the water, the copper dissolves to create a microscopic toxic cloud right against the fibreglass. Barnacles, tube worms and weed spores attempt to attach, absorb the biocide and die before they can take hold.
Many modern paints include a secondary biocide like zinc pyrithione. This targets the soft green slime and algae that copper alone struggles to stop. Owners of aluminium boats or outdrives must never use standard copper bottom paint. It causes severe galvanic corrosion. You need paints using copper thiocyanate (like International Trilux 33) or metal-free biocides like Econea.
Ablative Self-Polishing Paints
Ablative paints are the standard choice for sailing yachts and displacement motorboats doing under 15 knots. The resin holding the biocide is slightly water-soluble. As the boat moves, water friction washes the top layer of paint away. This constantly exposes a fresh layer of copper.
- Pros: You avoid a massive build-up of old paint over the years. Winter preparation takes less time.
- Cons: If your boat does 25 knots, water pressure strips the paint off the bow and waterline in weeks.
- Typical brands: International Micron 350, Hempel Mille NCT, Seajet 033 Shogun.
Hard Contact Leaching Paints
Hard bottom paints use a tough, insoluble resin binder. When submerged, seawater penetrates the porous surface of the paint. It dissolves the biocide and leaches it out. Once the biocide vanishes, the hard paint matrix remains on the hull like an empty honeycomb.