How do I choose the right antifouling paint for my boat and local waters?
I keep a 9-metre fibreglass cruiser afloat from April to October, mostly in a sheltered saltwater marina. There are several different antifoulings in the chandlery, and I don't know exactly what the previous owner used. How do I choose something suitable without having to strip the whole bottom?
Answer
Choose antifouling by checking your hull material and existing coating, then match the paint to local fouling, cruising speed and time afloat. For most sailing yachts and motorboats cruising below about 20 knots, self-polishing antifouling is the usual starting point; hard antifouling generally suits faster boats, frequent handling and racing finishes.
Check hull and paint compatibility
Even a suitable paint will fail if it attacks the hull material or cannot stick to the coating underneath. Find the previous owner's paint tin, invoice or maintenance record before buying.
- Fibreglass: most conventional antifoulings are suitable over a sound, compatible underwater primer and coating system.
- Aluminium: use only a product explicitly approved for aluminium, with its specified primer system. Conventional copper-containing antifouling can cause serious corrosion.
- Steel: the protective epoxy coating must be sound; antifouling does not replace corrosion protection.
- Wood: traditional soft antifouling is a common choice, but construction and previous coatings matter. Treat epoxy-sheathed timber as a complete coating system.
Treat aluminium saildrives, outdrives and propellers separately from the hull: their coating requirements can differ. Keep antifouling off anodes.
If the old paint is unknown but firmly attached, ask the new paint manufacturer whether its tie coat can isolate it. A tie coat cannot stabilise paint that is loose, blistered or heavily flaking. Remove those areas back to a sound surface.
Match paint to boat use
Self-polishing antifouling gradually wears away, exposing fresh active material. It suits most displacement cruisers and sailing yachts, usually with seasonal renewal. Some paint remains each year, so eventual stripping is still necessary.
Hard antifouling leaves its binder behind as the active ingredients are released. Its abrasion resistance suits fast motorboats, trailer contact and racing boats whose bottoms are carefully smoothed. Sand the exhausted coating before recoating, and control accumulated thickness annually or every other year.
Traditional soft antifouling erodes as its binder and active ingredients dissolve. It is commonly used on traditional wooden boats. Changing between soft, hard and self-polishing systems may require removal or preparation and priming specified by the manufacturer.
About 20 knots is a useful starting point, not a universal product limit. Check the paint's maximum speed, suitability for drying out and permitted time ashore after use.
Ask about local fouling
Ask owners on your pontoon which exact product worked through the previous season, how often they sailed and whether they cleaned the bottom. A regularly sailed yacht on an exposed mooring provides a different test from a boat sitting in a warm, sheltered marina.