Antifouling looks routine because it is done every year; yet the wrong paint or poor preparation leads to peeling, corrosion on aluminium parts, and double the cost the following season.
Types of antifouling
- Hard
- Stays on the surface without wearing away. Favoured on fast boats and boats hauled often; builds up in layers over time.
- Self-polishing (ablative)
- Wears slowly in the water to expose fresh surface; does not build up. Suits boats that stay afloat for long.
- Copper-free
- For aluminium hulls, saildrives and aluminium parts. Copper-based paint causes corrosion on these metals.
Compatibility and preparation
- Compatibility: can the new paint go over the old, or is a tie coat needed; the maker's compatibility chart decides.
- Built-up layers: thick, flaking old paint is removed completely, by blasting or scraping.
- Epoxy barrier: can be applied after stripping as osmosis prevention; the hull must first be shown dry by moisture readings.
- Data sheet: number of coats, overcoating times and the minimum and maximum time before launch follow the maker.
The sequence
- Wash and inspect. Blisters, flaking and bare spots are marked.
- Preparation. Sanding, stripping where needed; primer on bare spots.
- Masking. Waterline, anodes, transducers and log; anodes are never painted.
- Application. Coats per the data sheet; an extra coat at the waterline and the rudder's leading edge.
- Launch. Within the maker's launch window.
Continue with this topic
- Gelcoat repair and polish: Topsides and cosmetic repair.
- Paint systems in a build: Preparation, dew point and film thickness.
How often?
Antifouling life depends on the paint type, how often the boat is used, water temperature and how fast the boat goes. Fouling is faster in warm, rich waters. On many boats the paint is renewed every year or two. A mid-season clean by a diver can extend its life, but hard scrubbing wears self-polishing paint away faster than intended.
What the fouling pattern tells you
When the boat is hauled, how the bottom has fouled shows how well the paint worked. Heavy weed just below the waterline calls for an extra coat there; bare patches on the leading edges of rudder and keel show faster wear in those areas. One-sided fouling suggests the boat lay the same way for a long time. These observations set the number of coats and where they go next time.
Environment and rules
Antifouling, as the name says, contains active substances to keep marine growth away. Collecting the dust and waste from stripping and sanding is regulated at most marinas. Lower-impact products and biocide-free foul-release coatings are also options; the choice follows the boat's use and local rules.
If you do it yourself
Wear a dust mask, gloves and eye protection when sanding and painting. Ask the marina about its rules for owner work and waste in advance. Follow the overcoating and launch times in the maker's data sheet.
An inspection opportunity
Painting is one of the rare times the whole bottom is visible ashore. It is a good moment to look for osmosis blisters, impact marks, anode wear and the condition of through-hulls, before the new coats cover everything.
Findings then go into the same haul-out's work list, with no second lift needed.
A careful look before painting costs little and can save a second haul-out later.
Record the paint used, its batch and the date in the maintenance log.