How do I choose bottom paint for my boat and conditions?
I have a 27ft fibreglass sailing yacht that stays afloat from April to October, mostly in a sheltered saltwater marina. I don't know which antifouling the previous owner used, and I'm unsure whether to choose hard or self-polishing paint.
Answer
Choose bottom paint, usually called antifouling, to suit your hull material, local fouling, cruising speed and time afloat. Identify the existing coating and check its condition first: even the right antifouling can peel over incompatible or failing paint.
Check the hull and coating
Find the previous owner's paint tin, maintenance records or yard invoice before buying paint. Colour alone cannot identify a coating. Paint that rubs off on a wet cloth may be soft antifouling, but that test is only a clue.
- Fibreglass: most conventional yacht antifoulings are suitable over the specified primer or a compatible existing coating.
- Aluminium: use only a product explicitly approved for aluminium, including on saildrive legs and outboard lower units. Ordinary copper-based antifouling can cause serious corrosion.
- Steel: the anticorrosive primer system must be sound; antifouling does not replace it.
- Wood: check approval for the timber and construction method, particularly with traditional planked hulls.
If the old coating is firmly attached, check the new manufacturer's compatibility chart. Sound paint of unknown type may accept a specified tie coat. Remove loose or cracked paint back to a sound surface, and do the same where excessive layers have built up. A primer cannot glue failing layers together.
Match paint to boat use
Self-polishing or eroding antifouling is the usual starting point for a cruising yacht or moderate-speed motorboat kept afloat through the season. These coatings gradually renew their surface, limiting the build-up of exhausted paint. Their chemistry varies, so check suitability for your speed and periods without use.
Preparation can be straightforward: wash thoroughly, remove slime and loose material, and follow the overcoating instructions. Some systems need little sanding between seasons; others require abrasion. Neither annual sanding nor skipping it suits every self-polishing coating.
Hard antifouling suits boats needing a tougher surface, including some fast motorboats, frequently scrubbed hulls and racing yachts that require a carefully finished bottom. Check the maximum speed and whether burnishing or scrubbing is permitted. Hard paint leaves an exhausted film behind, so repeated applications eventually demand more substantial preparation.
Biocide-free foul-release coatings, often silicone-based, can suit owners prepared to follow a complete coating system and cleaning regime. Performance depends on the product, boat movement and local fouling. These coatings are not automatically a maintenance-free replacement for conventional antifouling.