Why is my boat's antifouling paint turning yellow?

Your boat's antifouling usually turns yellow because of surface staining, a thin film of growth, or a colour change as the coating reacts with water and air. Check whether the yellow wipes off and whether the paint underneath is firmly attached before deciding to clean, repaint or strip it.

Check the surface first

Start with a small patch near the waterline and compare it with an area farther down the hull. White and pale-grey antifoulings show yellow or brown discolouration much more clearly than dark blue or black coatings.

With the boat lifted, photograph the hull before washing. Rinse a small test area with fresh water, then wipe gently with a wet cloth or soft sponge. Avoid aggressive scrubbing: soft antifouling can come away with the stain, making it look as though you have cleaned it.

• Yellow material wipes away: suspect slime, sediment or staining deposited on the surface.
• The coating itself stays yellow: a change within the paint, or a persistent stain, is more likely.
• Yellow-orange marks start at chips, fittings or the keel: investigate corrosion beneath or above the coating.
• Paint lifts in flakes: adhesion or accumulated paint thickness needs attention, regardless of colour.

Wear gloves and eye protection when handling antifouling. Use the boatyard's approved facilities to collect contaminated wash water, scrapings and sanding residue.

Common causes of yellowing

Slime and waterborne deposits. A thin biological film can look yellow, mustard or brown before obvious weed appears. Mud, suspended sediment and organic material can also discolour the surface, particularly around the waterline or after a long spell unused. A slimy feel points towards this cause, although firmly attached deposits may feel smooth.

Changes in the coating. Some antifoulings change shade after immersion or exposure during a lift-out. Copper compounds, pigments and the paint binder can affect their appearance; the expected colour depends on the exact formulation. A uniform change across sound paint is less concerning than isolated patches with cracking or blistering.

Check the tin, invoice or previous owner's maintenance record for the exact product and colour. "International white" or "Hempel grey" is not enough: different products within one brand can behave differently. The manufacturer's technical support can confirm whether that particular colour normally changes in service.

Rust staining. Yellow-orange streaks may run down from a steel fitting or emerge through failed protection on a cast-iron keel or steel hull. Antifouling does not replace an anticorrosive primer. Painting over active rust only hides the problem temporarily.