How to prepare and apply antifouling

Rupert Holmes looks at how to get a long-lasting finish while avoiding the hazards of this toxic substance

27 April 2012.
By Rupert Holmes

Antifoulings have to do a tough job in many locations, so it makes sense to ensure they’re applied in the best possible conditions. As soon as a boat is hauled ashore the bottom should be pressure washed to remove any existing fouling – if this is allowed to dry it becomes many times more difficult and time consuming to remove. Small areas or loose or flaking antifoul can be removed with a paint scraper, however if many layers of antifoul have been allowed to build up and large areas are poorly adhered to the hull it makes sense to strip the hull back to a sound surface.

antifouling a yacht

Antifouling dust is extremely hazardous, so the only safe DIY options are wet sanding, or stripping with a chemical stripper that’s formulated for use on fibreglass. Even then full protective clothing, including goggles and an effective face mask is vital – a cheap decorator’s mask isn’t up to the job of protecting lungs from the poisonous dust. A better option is to use a soda or slurry-blasting contractor with experience of removing antifouling. Although needing specialist equipment, this is a much faster process so the savings in labour – and mess – are nearly always worthwhile, even for the most ardent of DIY boat owners.

Often the waterline can still be discerned even after all the antifouling has been removed. However before doing so it’s a wise precaution to measure the distance of the waterline below the gunwhale at intervals of a couple of feet – it will then be possible to accurately reinstate the waterline.

Preparing for antifouling

On removing the boat from the water any weed should be removed with a pressure washer before it has time to dry

Final preparation before antifouling

Good preparation and priming is essential to ensure that the antifouling adheres to the surface. In addition, that surface should ideally be as smooth as possible – the faster the boat the more important this aspect becomes. On any half-serious raceboat that means ensuring the hull is properly faired and abrading previous coats to prevent a build up of layers of paint.

Using masking tape to prepare yacht before antifouling

Apply masking tape at the waterline to keep a crisp straight edge, but removing it after each coat of antifoul – it can be difficult to remove if left in place - photo Rupert Holmes

Bare fiberglass should be primed with an antifouling primer appropriate to the type of antifoul used (see our feature How to choose the right antifouling). If the hull has been stripped back to the gelcoat it’s also worth considering applying epoxy coatings to guard against osmosis. If considering this it’s important to first get a surveyor to test the moisture levels of the hull as sealing excess water in with epoxy is counter productive.

With modern antifoulings there are now fewer problems with using a different type over existing coatings than was the case in the past. However, if you’re at all unsure it’s worth applying primer before the new antifoul as a tie coat.

Keels also need special preparation, both to ensure they are perfectly faired, and to give the antifoul the maximum opportunity to remained strongly adhered to the surface. Any rust on cast iron keels should be removed with an angle grinder, such that the surface of the metal is bright. It can then by primed and faired before application of antifouling.

With unpainted lead keels the tarnished oxide layer should be removed rubbing down with emery paper or with a wire brush in a power drill. Then remove the grease and contamination by washing with a suitable cleaner, before initial priming with an etch primer. This can then followed by the recommended primer for the antifoul used.


How many coats of antifouling are needed?

Unless the boat is to be kept afloat for a period of only a few weeks, or local fouling conditions are very weak, at least two coats should be applied, with a third around the waterline and edges of the rudder and keel of sailing boats. Don’t be tempted to stretch the paint out – for the antifouling system to be effective you need to apply as much biocide to the hull as possible. Minimum overcoating and immersion times should also be adhered to closely – failure to do so is one of the most common reasons for antifoul to start flaking away from the hull.

Although a tin of antifoul may not appear to require stirring when opened, it is always important to do so thoroughly – many products have more than 50 per cent solid content, which sinks to the bottom of the tin during storage. The stirring devices designed for use with a cordless drill are ideal for this purpose.

Applying antifouling with a roller

A smaller roller with a long handle is the easiest and least messy way to apply antifouling, but don’t forget protective clothing - photo Rupert Holmes


Applying antifouling

The quickest – and least messy – method of applying antifouling is to use a small medium pile roller with an extension handle. This keeps you at arm’s length from the paint and minimises the amount of bending and stooping necessary. It can be tempting to use a larger roller, but these absorb a large amount of paint and with antifouling being expensive the time saved rarely justifies the additional cost, other than on the largest of yachts. Professional application can also be by spraying, but specialist facilities are essential to avoid the risk of breathing harmful vapours and toxic particles.

As well as accurately masking the waterline, don’t paint zinc sacrificial hull or shaft anodes, as this will wreck their performance. Similarly, remember that aluminium outdrives may need a different type of antifoul to the hull.

When applying antifouling a face mask, overalls, gloves and goggles are needed to protect against contact with the paint. If your overalls don’t have a hood it’s worth wearing a hat – it’s surprisingly easy to rub your head against the underside of the hull, with embarrassing and toxic results.

For details on selecting the right antifourling see our feature How to choose the right antifouling.




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