Antifouling Paint Explained: Hard vs Ablative and How to Do the Job

What Fouling Costs You

A hull in the water starts fouling the day it is launched. First a bacterial film, then slime, then weed, then the hard growth — barnacles, tube worms, mussels — that cements itself on. Each stage adds friction, and friction is most of a boat's resistance at low speed. Even a slime film, the stage owners shrug at, raises fuel burn under power by an amount that shows up on the gauge over a season. Heavy fouling on a sailing boat commonly costs 10–20 percent of her speed and sometimes more; the exact figure depends on the hull shape, how much of the wetted surface is affected and the wind strength, because in light airs skin friction is nearly all the drag there is. The symptom is a boat that will not point, will not tack cleanly and needs the engine to make a passage she used to sail.

The other cost is sharper. A fouled keel and rudder stall earlier, so the boat handles worse exactly when you need her to: coming alongside, or clawing off a lee shore. A fouled propeller loses thrust faster than a fouled hull loses speed, and a fouled engine intake overheats it. Antifouling is not cosmetic. It is the cheapest performance upgrade and the cheapest safety margin the boat gets each year.

The Biocide: Copper, Zinc and the Rules

Nearly all conventional antifouling works the same way: a resin binder holds a biocide, and the biocide leaches into the thin layer of water against the hull at a rate slow enough to last a season and high enough to stop larvae settling. For most of the twentieth century the biocide of choice was tributyltin, TBT, which was superb at the job and disastrous for everything else in the harbour. It was banned globally under the IMO International Convention on the Control of Harmful Anti-fouling Systems on Ships, which came into force in 2008, and you cannot buy it.

Today the workhorse is copper — cuprous oxide in most paints, or copper thiocyanate in the lighter colours, since cuprous oxide is dark red-brown and drags every shade towards it. Copper is backed up by an organic booster biocide aimed at the slime and weed that tolerate copper. Zinc compounds are the usual biocide in copper-free paints; they are gentler on aluminium and adequate in moderate fouling.

Copper itself is now under pressure. Washington State has legislated a phase-in of restrictions on copper antifouling for recreational boats; some Baltic marinas and Scandinavian authorities limit or forbid copper paints in their waters; parts of Europe require registered, approved products only. The picture varies by country, by state and sometimes by harbour, and it changes — so the honest advice is: check locally before you buy, and buy from a chandler in the country you will haul out in, because the approved list is theirs, not yours.

Hard, Ablative, Hybrid, Silicone, Copper-Free

The table above is the summary; here is the reasoning, because the reasoning is what tells you which one is yours.

Hard, or contact-leaching

The binder is insoluble. Biocide leaches out from the surface, leaving behind a honeycomb of spent paint that stays on the hull. That gives you a film you can scrub, burnish, and leave sitting in a berth for months without it going anywhere — and a film that gets thicker every year until, four to six coats on, it cracks and lets go in sheets and you spend a fortnight with a scraper. Hard paints are also the ones with a launch window: many must be in the water within a set number of days of application or the surface oxidises and stops leaching. Read the tin. Some say seventy-two hours.

Ablative and self-polishing

The binder dissolves slowly in seawater, so the surface wears away and exposes fresh biocide all season. There is no build-up and the hull comes out of the water with little on it but a thin, even film. The catch is in the word wears: it needs water moving over it. A boat that spends the summer in a still marina does not polish its ablative; it grows slime on it. And a crew that scrubs an ablative hull mid-season is scrubbing off the paint. Self-polishing copolymers, SPC, are the refined version: the erosion rate is chemically controlled rather than simply dissolving, which is why they cost more and why a good one on a well-sailed boat can go two seasons.

Hybrid

A partly eroding film that tolerates a light wipe. It exists because most boats are neither racers nor liveaboards but a bit of each, and it does a fair job for both without excelling at either.

Silicone foul-release

No biocide at all. The surface is so slick, at such low surface energy, that fouling cannot get a grip and sloughs off once the boat is moving. On a planing powerboat or a hard-sailed racer it is close to magic and lasts several seasons. On a boat that sits, growth attaches anyway — loosely, but it attaches — and has to be wiped. Two more catches: it is expensive, and nothing sticks to it, including the next coat of anything else. Going to silicone is a commitment, and coming back means stripping to the barrier coat.

Copper-free

A hard or ablative binder with a zinc or organic biocide. Not a type so much as a requirement — for aluminium, for outdrives and for waters where copper is banned — and the reason comes next.

Aluminium Boats and Outdrives: Copper Is Corrosion

Copper is more noble than aluminium. Put a copper-loaded paint on an aluminium hull, an outdrive leg, a saildrive or an outboard and you have built a galvanic cell with seawater as the electrolyte. Wherever the primer is scratched — and it always is — the aluminium goes first, as a white powdery bloom and then as pitting. This is not a matter of preference or a slightly worse result. It is the aluminium dissolving. Use a copper-free paint over a maker-specified aluminium primer, and use it on every aluminium part below the waterline, including the leg of a sterndrive on a fibreglass boat whose hull is happily wearing copper. Most makers sell a dedicated outdrive spray for this.

For the same reason, keep every kind of antifouling off the sacrificial zincs. A painted anode is a decoration; it protects nothing.

Can You Antifoul Over Old Antifouling?

Usually, and this is where the fence argument turns into a flaking hull. The rules of thumb:

• Ablative over hard: fine, after a pressure wash, a light sand to key the surface and a solvent wipe.
• Hard over ablative: usually not. The soft layer keeps moving under the hard film and the hard film cracks and lifts. Some makers allow it with a sealer coat; most say strip.
• Anything over silicone: no. Nothing sticks to it.
• Silicone over anything: no. It goes on its own tie-coat over a fresh barrier coat, on a hull taken back to epoxy or gelcoat.
• Any paint over flaking, chalky or unknown paint: strip. New paint is only as well attached as the worst layer beneath it.

Every maker publishes a compatibility chart — their paint over their paint, their paint over competitors', and what primer or tie-coat goes between. Read it before you buy, not after. If you are not sure what is on the hull, a test patch of the new paint on a hand-sized area, left a week and then picked at with a thumbnail, is worth the wait.

Your Boat, Your Paint

This is the thesis in a table. Find your row, and notice that the "best" paint on the fence is probably in somebody else's.

How to Antifoul a Boat: the Job Step by Step

The paint is a day. The preparation is the rest of the week, and the preparation is what decides whether the paint lasts.

1. Haul out and pressure wash immediately, while the hull is wet. Slime and soft weed rinse off in minutes when wet; left to dry for an afternoon they set like plaster. Get the yard to wash while the boat is still in the slings.
2. Scrape and sand — in a mask. Old antifouling dust is biocide, and it is designed to kill things. Wear a proper particulate respirator, gloves, goggles and a disposable suit; wet-sand where the yard allows, and collect the residue for the yard's waste. Take off anything loose. If the surface is sound you need only a light key; if it is thick, cracked or flaking, this is the year to strip to the barrier coat.
3. Tape the waterline. Come up 25–50 mm (1–2 in) above the static waterline; a boat loaded for cruising sits lower than she did new, and the first few centimetres above the water foul worst because they are wet, warm and lit.
4. Barrier coat if bare. Any gelcoat or bare laminate showing gets an epoxy barrier coat first, for osmosis protection and as the primer the antifouling needs. On aluminium, the aluminium primer. Follow the overcoat times on the tin exactly; they are chemistry, not advice.
5. Two coats, and a third at the leading edges and waterline. The stem, the leading edge of the keel and rudder, the skeg and the waterline band wear first. Change colour between coats so you can see where the first has worn through next year — that tells you where to put the extra coat the year after. Roll; do not spray unless the yard is set up for it.
6. Do not paint the anodes, the transducer face or the propeller shaft. A painted anode does nothing, a painted depth-sounder face reads badly, and antifouling on a turning shaft does not stay on anyway. The propeller itself takes a dedicated propeller paint if anything.
7. Launch inside the paint's window. Ablatives are relaxed about it. Many hard paints must be in the water within days — the tin will give a maximum — or the surface skins over and the season is lost. Book the launch before you open the first tin.

For quantity, a rough guide is that one litre covers around 8–10 m² per coat; the maker's figure for your paint, times your wetted area, times the number of coats, plus ten percent for the leading edges. A 10 m (33 ft) sailing boat typically uses 2.5–4 litres for two coats. Write down what you actually used — next year's order is then a fact rather than a guess.

Seasons, Waters and How Often

Fouling is a function of water temperature, nutrients, light and stillness. A Mediterranean marina in August — warm, still, sunlit and full of nutrients — is close to a laboratory culture, and boats that sit there through summer foul faster than the same boats sailing between anchorages. The tropics foul year-round. Cold northern water fouls slowly through winter and hard in a short summer.

So the intervals in the tables are honest averages. Annually in most of the Mediterranean and Caribbean and in the warm-water United States. Two seasons in cold water, or in any water with a premium SPC on a boat that sails most weeks. The real test is the hull, not the calendar: a snorkel or a diver every couple of months, and if the growth is more than a soft slime that wipes off with a hand, the paint is finished whatever the tin promised.

On in-water cleaning: many marinas forbid it outright, because scrubbing a biocidal paint puts a season's worth of copper into the berth in an afternoon, and some jurisdictions regulate it. Where it is allowed, wipe with a soft pad, never a scraper, on an ablative, and know that you are shortening its life. On silicone a wipe is the maintenance and takes ten minutes.

Keeping the Record

The hull tells you what worked, but only if you remember what you did. Which paint, which colour underneath, how many coats, how many litres, when she went back in and when the anodes were changed — those six facts are the difference between a considered choice next spring and another argument on the fence. Keep them in the boat's maintenance log, under the haul-out date, with a photograph of the tin. The Marine Logbook has a maintenance section built for exactly this: a haul-out entry with the paint brand and colour, coats and litres, and the anode dates alongside, so that when the yard asks "what is on her?" you can answer from your phone rather than from memory. Whatever you use, use something. The paint you cannot identify is the paint you have to strip.

The Bottom Line

Antifouling is chosen by how you use the boat, not by which paint is best. A hull that sails every weekend wants an ablative that polishes itself clean; a hull that sits in a warm marina wants a hard paint a diver can wipe; a planing hull wants hard or silicone; anything aluminium wants copper-free, without exception. Check the compatibility chart before you buy, wash the hull the hour she comes out, sand in a mask, put the third coat on the leading edges, leave the anodes bare, launch inside the window — and write down what you did, because next year's right answer starts with this year's facts.