Why is my boat's bottom paint peeling or coming off?

Your boat's bottom paint is usually peeling because old layers have lost their grip, the surface was poorly prepared, or the latest antifouling is incompatible with the coating beneath. Check which layer is separating first to judge whether a local repair will hold or the bottom needs stripping back to sound primer.

Identify the failing layer

Inspect the hull after washing and drying, before sanding hides the evidence. Examine the exposed patch and the underside of a loose flake. Then gently test the surrounding edges with a scraper held nearly flat.

• Only the newest colour lifts: suspect contamination, inadequate preparation, incompatibility or a missed overcoating window.
• Several colours come away together: the weak layer lies further down, often beneath years of accumulated antifouling.
• Primer comes away and exposes gelcoat: investigate preparation and adhesion beneath the primer. Another antifouling coat will not fix it.
• Orange staining or rusty flakes appear: corrosion is lifting the coating on a cast-iron keel or steel hull.

Self-polishing antifouling gradually wearing thinner, or leaving coloured residue during washing, is normal. Sheets, curled flakes and sharply edged bare patches indicate coating failure. An aggressive pressure-washer jet can also remove otherwise serviceable soft paint, particularly at close range.

Why adhesion fails

Excessive build-up is a common cause. Self-polishing paint does not disappear completely each season. Residual layers accumulate until the stack cracks and detaches. Complete removal back to sound primer is often needed after roughly 5 to 15 years, depending on application thickness and use. Hard antifouling retains more of its coating and generally needs sanding annually or every second year to control build-up.

Poor preparation leaves a weak bond. Slime, salt, grease, loose pigment and sanding residue can all prevent adhesion. On previously unpainted GRP, remove mould-release wax with the specified cleaner before abrasion. Sanding first can spread contamination.

Changing systems needs a compatibility check. Switching brands is not automatically a problem, but changing paint technology can be. Conventional antifouling will not reliably adhere to silicone foul-release coatings. A hard coating over a weak, soft antifouling can also fail because the underlying layer cannot support it.

Application conditions matter too. Condensation, a damp hull, excessive film thickness, incorrect thinning or launching outside the permitted interval can spoil adhesion or curing. Follow the exact product's temperature, overcoating and immersion limits. There is no universal overnight drying rule.