DIY antifouling: one hull of a job. Keep reading to see how to work out the correct amount to use.

DIY antifouling - Hull of a job

I’ve been antifouling my own boats for two decades now. Those more sensible than me employ professionals to do the job for them, but it pays to shop around and ask for a detailed list of exactly what you’re getting for your money. Make sure you know what the shipwright or painter intends to do and get an estimate or quote before going up on the hard.


DIY antifouling

If you are in the leagues of Do-It-Yourself antifoulers your first step is to work out what products you require and in what quantities. There are some simple calculations for this. For a rough estimate, when buying paints, I purchase one litre of paint for every 4m² of hull — that’s per coat.

I then use paint supplier Altex’s simple equation to help me work out how many square metres of hull I will need to paint:


* For displacement hulls the equation is Length of Waterline x (Breadth + Depth) = square metres.* For semi-displacement hulls it is Length of Waterline x (Breadth + Depth) x 0.75 = square metres.

* And for deep-keeled racing yachts the rule of thumb is Length of Waterline x (Breadth + Depth) x 0.5 = square metres.


Once you have worked out your equation for how much antifoul you require per coat don’t forget you will need to buy enough for at least two full coats plus another coat for the trailing edges. Leading and trailing edges are high-wash areas such as the first half metre or so down from the waterline and areas including the keel, rudder, prop and bow. Record in your logbook all relevant details, paint volume, type of paint, number of rollers, brushes, etc that you require. Also, record the names and phone numbers of your local suppliers. Trust me, by the time the next antifoul comes around you will have forgotten.

It’s a bad idea to put some paint aside for the following year. Antifoul doesn’t stop marine growth if it’s left in the tin and paint suppliers do warn that it can go off. And if you don’t put enough paint on you also risk premature failure.



Shipwright Ray Pettengell of R & B Marine (Mackay, Qld) says if you have any concerns about your hull bring in an expert before you go ahead with the antifouling.

“Once your boat gets put in the cradle you will need to assess whether or not there is other work to do,” said Ray.

“The best time to pick out any problems is after the water blast. It’s then you can easily pick out, for example, osmosis or electrolysis or any other type of mechanical damage. Any areas of concern must be addressed first,” he said.

Ray also says it’s a good time to check the play in your rudder and prop bearings and also to inspect all skin fittings.

“After any concerns have been properly addressed and your hull prepared, you will need to wet and dry down the surface area and re-prime the bare patches,” he said.

“However, if your hull is in good condition and you only need to recoat the antifoul then your boat must still be sanded because the depleted antifoul, which is green in colour, must be removed.

“Once that is removed, if you know what paint you have had on the boat previously, you can recoat with the same product. But you must consult a paint company or a shipwright about that,” Ray says.

Paint supplier International recommends wet sanding with 80 grit paper to remove the top layer of paint that is depleted in biocides and contains salt and calcium deposits. If this is not done then the antifoul may either blister, fall off or the biocides of the old paint could permeate into the new paint causing a depleted layer. Don’t forget that any edges left by removal of scraped paint need to be sanded smooth and feathered back into the surrounding sound paint system. Also, you will need to stir the antifoul thoroughly before use as it contains heavy compounds that can settle to the bottom of the can.


Using a sealer coat

If you don’t know what product was on your boat previously then it’s advisable to properly prepare the hull first, then to seal off your hull with a tie/barrier coat before antifouling.

There is some great advice on paint supplier’s websites, plus it pays to phone or drop into the paint companies and get the correct technical advice from their reps. All paint suppliers agree that it is important paint products be mixed and applied in accordance with their labels or data sheet instructions and/or advice given by their authorised agents.

As Ray Pettengell says, “a tie coat or a barrier coat will seal the previous surface and make a good, decent surface for the antifoul to adhere to.

“All paint companies, if they sell you paint, should give you a data sheet and in that data sheet they will tell you exactly what sort of minimum times you must leave between over-coating and at what temperature… and you must be really strict about that.

“The other thing you must remember is if you leave the boat overnight then you should wash down the next morning and allow it to dry thoroughly before carrying on.

“The reason for this is, overnight, the salt air may leave deposits of salt on your hull and if you antifoul over that salt deposit it could lead to problems with adhesion,” says Ray.


Antifouling: frequently asked questions 

Do Aluminium boats, outboards, sail drives etc require a different antifoul?

Yes. The simple rule of thumb is to check whether the antifoul is suitable for aluminium. Under no circumstances should you ever apply antifoul to bare aluminium. I’ve seen this done and it will cause damage. It is very important to establish what type of metal you’re working on and to choose the best primer, barrier coat and compatible antifoul for that material.


What is the purpose of zinc anodes?

Zinc anodes are a less noble metal and if attached properly should deteriorate protecting any metal fittings they are connected to below the waterline. They must be correctly attached and should never be antifouled over. Sounds simple but I’ve seen it done wrong many a time. They’re called sacrificial anodes for a reason.


What about protective gear?

If you are planning to antifoul your own boat then it’s important to wear protective clothing and eye wear and use protective equipment to prevent paints and thinners coming into contact with your skin and eyes. Also, don’t forget to use good quality respirators or dust masks.

Altex recommends the use of barrier creams but stress this is not a substitute for gloves and masks. Should you splash paint on your skin it’s best to use soap and water or an industrial skin cleaner to remove it. Disposal overalls are a wonderful thing along with good quality latex gloves and a respirator.


In perspective...

Nobody I know looks forward to a weekend on the slipway. But spare a thought for the guys who painted one of the world’s biggest cruise ships Oasis of the Seas. A total of 630,000lt of Jotun paint was applied to this luxury vessel.

Now that’s one boat you wouldn’t volunteer to paint with a roller.