Why is my antifouling paint not working?

Antifouling fails because the wrong paint was chosen for your hull speed, or it was applied too thinly to release biocides effectively. To solve the problem, match the paint chemistry to your local water conditions and apply the exact film thickness recommended by the manufacturer.

Erodible versus hard paints

A mismatch between paint type and cruising speed guarantees a fouled hull. Antifouling falls into two distinct categories.

• Erodible paints: Brands like International Micron 350 wear away gradually as the boat moves. This exposes a fresh layer of biocide. They suit sailing yachts and displacement motorboats cruising under 15 knots. Apply this to a 30-knot planing hull and water pressure washes the paint off in weeks. The hull remains bare and unprotected.
• Hard paints: Products like Hempel Hard Racing do not wear away. The biocide leaches out of a porous shell. Fast motorboats, boats on drying mud berths, and trailered vessels require this finish. If you use hard paint on a slow sailing yacht, the outer biocide layer depletes quickly. Without speed to scrub the surface, the paint clogs and stops working.

Application and film thickness

Stretching a 2.5-litre tin of paint too far is a false economy. Antifouling relies on a specific wet film thickness to hold enough copper for a full season. Roll it on like decorative house paint and it runs out of active ingredients by July.

Apply two generous coats to the entire hull. A third coat is mandatory on high-wear areas like the waterline, the leading edge of the keel, the rudder, and the bow. Applying paint late in the afternoon when the dew point drops traps moisture under the film. This compromises the cure and prevents the release of biocides after launch.

Idle boats and local waters

Antifouling requires water flow to work. If your boat sits idle on a marina pontoon for three months, slime and weed will form. Erodible paints rely on water friction to slough off dead layers and expose fresh copper.