How do I prepare and apply bottom paint to a boat?
My fibreglass yacht is ashore and the old antifouling is patchy, with a few flaking areas around the keel. How much should I remove, where do I need primer, and how many coats should I put on before launching?
Answer
Clean the underwater hull, remove loose antifouling, repair damaged areas and prime exposed surfaces before applying a compatible bottom paint. Sound, firmly attached paint normally needs preparation and recoating; a fibreglass hull does not need stripping back to gelcoat every season.
Check the existing coating
Identify the old antifouling from previous invoices, tins or the boat's maintenance record. Compatibility determines whether you can paint straight over it, need a tie coat or must remove it.
After pressure washing, inspect the waterline, keel joint, rudder and patches beneath cradle pads. A scraper should remove loose edges without lifting sound paint around them. Widespread peeling, deep cracking or thick, uneven layers justify stripping. Isolated defects usually need local preparation.
Choose paint to suit your hull material, cruising speed, mooring conditions and time afloat. Eroding antifouling suits many cruising yachts. Hard antifouling may suit faster boats or hulls that are regularly scrubbed, subject to the product's speed limits.
For aluminium hulls and saildrives, use antifouling explicitly approved for aluminium over its specified primer system. Ordinary copper-based hull antifouling can cause serious corrosion.
Wash and prepare the hull
Pressure wash soon after lifting, while weed and slime are still wet. Remove barnacles and their attached bases, wash away contamination and let the hull dry.
Wear gloves, eye protection and suitable respiratory protection. Collect paint residue. Never move supports or work beneath an inadequately supported boat.
- Remove failed paint. Use a scraper with rounded blade corners to avoid gouging the hull. For extensive removal, choose a stripper approved for the hull material or employ a specialist.
- Feather sound edges. Wet abrade existing antifouling where the manufacturer permits it, using the specified abrasive grade. Avoid uncontrolled dry sanding and capture contaminated wash water.
- Protect the gelcoat. Stop at sound primer or gelcoat when stripping a fibreglass hull. Exposed glass fibres mean you have gone too far.
- Clean the prepared surface. Remove sanding slurry and dust using the paint maker's specified method. Use solvent only where the coating instructions require it.
Investigate blisters, persistent damp patches and movement at the keel joint before painting. Antifouling will not repair those faults.
Repair and prime bare areas
Fill small gouges with an underwater-rated epoxy filler, let it cure and sand it fair. Prime exposed gelcoat, filler and metal according to the chosen coating system. Steel and cast-iron keels require their own rust-removal and primer procedure.