Why does my boat's bottom paint keep peeling, and how can I prevent it?
My boat's antifouling comes off in flakes when I pressure-wash it, exposing different coloured layers underneath. I've repainted it each spring, but the peeling returns. How far back should I strip it, and what will stop it happening again?
Answer
Your bottom paint usually keeps peeling because the new antifouling sits on weak old layers, a contaminated surface or an incompatible coating. To stop it, identify where adhesion has failed, remove loose paint back to a sound surface, then prepare and recoat with a compatible system within the specified application times.
Find the failed layer
Before sanding, inspect several flakes and the patches they leave behind. The exposed surface helps you judge whether local repairs will suffice or a complete strip is needed.
- Several colours come away together: an older layer has failed beneath the latest coat. Painting over it adds weight without restoring adhesion.
- Only the newest colour peels: suspect poor preparation, incompatible products, dampness or a missed overcoating window.
- Clean, shiny primer or gelcoat appears: suspect inadequate abrasion, contamination or an unsuitable primer beneath the antifouling.
- Orange staining or rusty metal appears: corrosion is lifting the coating, commonly on cast-iron keels.
Self-polishing antifouling should gradually erode, often leaving a coloured smear when rubbed wet. Sheets and brittle flakes indicate adhesion failure, not normal polishing. Blisters in the underlying gelcoat need separate investigation; peeling antifouling alone does not establish osmosis.
Why repainting makes it worse
Self-polishing paint does not disappear completely each season. Residual material builds up until the weakest layer can no longer hold the layers above it. Complete removal back to sound primer is commonly needed after 5 to 15 years, depending on use, application thickness and paint type.
Hard antifouling leaves more coating behind. Sanding every year or every second year helps control buildup, but a thick, cracked stack eventually needs stripping.
Switching brands is not automatically a problem, but switching paint technologies can be. Conventional antifouling, silicone foul-release coatings and epoxy-based systems need different preparation. A tie-coat can bridge some approved combinations. It cannot secure loose paint or make every coating compatible.
Other common causes include salt left after washing, wax or grease, painting over condensation, excessively thick coats and launching before the paint has dried sufficiently. Check the product instructions: drying and immersion times change with temperature.
Remove failed paint safely
For isolated damage, scrape until the surrounding edges remain firmly attached, then feather those edges smooth. If several areas are peeling, or fresh edges keep lifting as you scrape, stripping the accumulated antifouling is usually more reliable than patching.