How to prepare and apply antifouling paint
Rupert Holmes looks at how to get a long-lasting finish while avoiding the hazards of this toxic substance in his guide to antifouling application.
March 19, 2021
When it comes to painting your boat, antifouling is perhaps one of the most dreaded taskes. The choice of antifouling paint as well as its application is important. Antifoulings have to do a tough job in many locations, so it makes sense to ensure they're applied in the best possible conditions.
How to apply antifouling paint:
- Use a small-medium pile roller with an extension handle
- Professional application can also be by spraying, but specialist facilities are essential to avoid the risk of breathing harmful vapours and toxic particles
- 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 due to the toxic nature of the paint
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 have poorly adhered to the hull it makes sense to strip the hull back to a sound surface.
Antifouling dust is extremely hazardous. 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. Don't be tempted to use a cheap decorator's mask as it isn't up to the job of protecting your lungs from the poisonous dust. A good option is to use a soda or slurry-blasting contractor with experience of removing antifouling. This is a much faster process so the savings in labour are nearly always worthwhile.
Often the waterline can still be discerned even after all the antifouling has been removed. However, before doing so it's worth measuring the distance of the waterline below the gunwhale at intervals of a couple of feet so you can accurately reinstate the waterline.
Final preparation before you apply 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.
Bare fibreglass should be primed with an antifouling primer appropriate to the type of antifoul (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 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. Also apply a third coat 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 as failure to do so is one of the most common reasons for flaking antifouling.
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 percent 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.
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. 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 as 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 antifouling see our feature How to choose antifouling paint. Looking for antifouling aternatives? See Coppercoat: a worthy alternative to traditional antifouling?