Why does my boat's bottom paint have black spots, and how should I treat them?

Black spots on a boat's bottom can be surface fouling, an older coating showing through, or dirt lodged in tiny pits in weathered gelcoat. Identify the affected surface and wash a small test patch: spots that survive 1200-grit wet sanding need closer inspection before you remove more material.

Identify antifouling or gelcoat

Locate the actual waterline and identify the finish. Below it, most conventionally painted cruising boats have antifouling over a primer; above it, a pale, glossy surface is often gelcoat or topside paint. Some boats have exposed gelcoat below the waterline, so position alone does not identify the material.

Examine an existing chip or worn edge without cutting into the coating. Several coloured layers suggest a paint system. A relatively thick, uniform white or cream finish may be gelcoat, although painted hulls can look similar.

• Dark material that wipes or brushes off: likely surface contamination or biological fouling.
• Flat dark patches matching a lower paint layer: likely wear-through or exposed older antifouling.
• Tiny dark pinpricks in a chalky, pale surface: consistent with dirt trapped in weathered gelcoat.
• Raised spots, cracking or loose edges: investigate coating failure or blistering before recoating.

If the spots occur mainly on one side above the waterline, long-term sun exposure at the berth can explain uneven gelcoat deterioration. That explanation is much less convincing for a permanently submerged surface.

Clean a test patch

Choose a patch about 100 × 100 mm and photograph it before cleaning. Work through these checks, stopping when you identify the cause.

1. Wash with fresh water and a cleaner approved for the existing finish, using a soft nylon brush.
2. Rinse thoroughly and inspect both wet and dry, preferably with a magnifying glass.
3. Check whether each mark sits on the surface, inside a small depression, or beneath a thinning topcoat.
4. If the coating is antifouling, check its product instructions before scrubbing harder or using chemicals.

Wear gloves and eye protection. Collect antifouling washings and sanding residue, and use suitable respiratory protection if the work generates dust.

Avoid experimenting with bleach, strong acids or solvents on an unidentified coating. They may alter the colour or damage the finish without treating the cause.

Treat weathered gelcoat

Old, chalked gelcoat can develop minute craters that hold dark dirt. Cleaning may remove dirt from the surrounding surface but leave these specks, which explains why a light pass with 1200-grit wet-and-dry sometimes achieves nothing.