Why does my boat's bottom paint keep peeling off?
The antifouling on my fibreglass sailing boat comes off in flakes every time it is pressure-washed, despite repainting each spring. Some patches expose another colour and others reach the primer. Do I need to strip the whole bottom?
Answer
Your boat's bottom paint usually peels because old antifouling has lost its grip, the surface was poorly prepared, or the coatings are incompatible. Check which layer is separating first: painting over failing paint gives the next coat a loose surface to stick to.
Read the flakes
After lifting and washing, let the hull dry and inspect several peeling patches, including an apparently sound area beside them. Work a scraper from an existing loose edge to find how far the failure extends, taking care not to gouge the coating beneath.
- Several colours come away together: the weakness lies within the old paint layers or where they meet the primer.
- Only the latest coat lifts: suspect preparation, compatibility or application conditions.
- Clean primer appears beneath the flakes: check adhesion between antifouling and primer, including whether the primer's overcoating window was met.
- Bare gelcoat or metal appears: the primer or complete coating system has failed locally.
- Rust or raised blisters sit beneath the paint: investigate the substrate before repainting.
Self-polishing antifouling gradually wears away and may release coloured residue during washing. Broad flakes, curled edges and sheets lifting from the hull indicate adhesion failure rather than normal polishing.
Old paint builds up
Self-polishing antifouling leaves some material behind each season. Annual repainting eventually builds a thick stack whose lower layers can no longer support the paint above. Look for cracking, a network of raised edges and flakes containing several colours.
A complete strip-back may be needed after roughly 5 to 15 years with self-polishing antifouling, depending on application thickness, use and previous maintenance. This is a maintenance range, not a fixed expiry date.
Hard antifouling does not polish away in the same way, so accumulated paint needs more deliberate removal. Sanding annually or every second year helps control build-up. Follow the product's preparation instructions.
If scraping a small patch keeps exposing more loose edges, stop patching. Remove the failing antifouling across the affected area, potentially the entire bottom, until you reach firmly bonded primer. Sound primer can usually stay if it suits the replacement system.
Check compatibility and application
A different brand is not automatically incompatible, but changing paint technology needs care. Hard antifouling over a weak polishing coating, conventional antifouling over silicone foul-release, or epoxy over old antifouling can all cause problems.