Bottom paint keeps weed and barnacles off the hull by releasing a biocide, often copper, into the water. The choice comes down to how the boat is used, what the hull is made of, and what's already on it.
Ablative or hard
| Ablative | Hard | |
|---|---|---|
| How it works | Wears away gradually, releasing more biocide as the boat moves | Stays in place as a film |
| Over the years | Wears thin, so you add more | Builds up in layers that eventually need blasting off |
| Suits | Boats used often; self-polishing types for boats that sit | Racers and fast boats scrubbed often |
| Recoating | Only when it wears thin | Single-season paints every season |
That's Soundings' summary. Practical Sailor puts the long-term difference in a line: ablatives wear away, hard paints "form thick layer cakes". It also warns that some paints, typically hard ones, lose their effect if the boat is hauled and relaunched without repainting, and need a light sanding to reactivate.
Some products sit between the two. Soundings describes Interlux Bottomkote NT as combining hard and ablative properties, and Trilux 33 as a slow-polishing multi-season ablative that holds up on fast powerboats.
Aluminium and steel hulls
Copper and aluminium don't mix. Interlux says copper oxide antifouling can go on a hull of any material except aluminium, and Soundings explains why: the copper reacts galvanically with the aluminium, and the aluminium loses. Aluminium owners usually choose a metal-free biocide such as tralopyril, sold as Econea, which Soundings says works on any hull and on underwater metal. Practical Sailor adds that aluminium saildrives need copper-free paint too.
Steel hulls can take copper oxide paint over the right primer. Practical Sailor notes that bare fibreglass or metal needs a primer.
Changing paint type
Practical Sailor's first rule is to choose a paint compatible with what's already on the hull, unless you're ready for more than a light sanding. A hard paint can usually be covered with either type. An ablative needs more sanding or a tie-coat before a hard paint goes over it. The makers publish compatibility guidance.
How much paint
Interlux gives a rough estimate of the underwater area: length overall × beam × 0.85. Divide by the coverage of the paint you've chosen, and multiply by the number of coats; Interlux's quick chart assumes two. Our antifouling paint calculator does it in feet or metres.
What a yard charges
| Yard | Bottom paint labour | Included |
|---|---|---|
| Huckins Yacht, Jacksonville | $26 per foot up to 39 ft, $28 up to 49 ft, plus material | Six hours of sanding; haul, block and launch charged separately |
| Liberty Landing Marina | $20 per foot plus paint and materials | $21.50 per foot with your own paint |
Add the haul-out: see boat haul-out cost for yard prices per foot in the US and France.
What the surveyor looks at
Out of the water, the surveyor sees the bottom before it's painted: blisters, cracks, the condition of the old coating and the metal fittings under it. See osmotic blistering and the out-of-water survey. With Axsail the surveyor dictates each finding under the hull, with its photo, while the boat is in the slings.