How do I prepare and antifoul a boat correctly?
My 32-foot fibreglass cruiser is coming ashore, and the antifouling is patchy with a few flaking areas around the waterline. How much old paint should I remove, where do I need primer, and how many coats should I apply?
Answer
Check that the existing antifouling is firmly attached and compatible with your chosen paint to establish whether it needs light preparation, a tie coat or complete removal. Wash off fouling, remove loose paint, repair and prime exposed areas, then apply the manufacturer's specified number of coats at the stated coverage and within its launching window.
Check the existing coating
Pressure-wash the hull as soon as the boat comes ashore, while slime and weed are still wet. Use the boatyard's designated wash area with runoff collection. Dried fouling takes much more effort to remove.
Once the hull is dry, inspect the waterline, keel, rudder and areas around skin fittings. Scrape several suspect patches. Sound paint should stay attached rather than lift in sheets.
- Sound, smooth paint with little build-up: clean and prepare it as the new paint's instructions require. There is no need to strip it all off.
- Local flaking: scrape back to firmly attached coating and feather the edges.
- Widespread cracking, flaking or thick ridges: remove the failing antifouling back to sound primer.
- Unknown paint: check the new manufacturer's compatibility guidance before buying. An approved tie coat may be necessary.
Self-polishing antifouling leaves some residual paint each season, so substantial removal may be needed after roughly 5 to 15 years. Hard antifouling builds up more persistently and generally needs sanding annually or every second year. Judge the coating by its condition, not its age.
Prepare without damaging the hull
Have the yard confirm that the boat is securely supported and leave the props untouched. Wear gloves, eye protection and the respiratory protection specified for the dust or solvent involved.
Lay sheeting beneath the working area and collect scrapings and slurry for the yard's designated waste stream. Never dry-sand antifouling without suitable dust extraction. Wet abrasion is often appropriate where the yard permits it.
- Remove loose material with a scraper, keeping its corners from gouging the surface.
- Abrade the remaining sound coating as instructed. Wet abrasive of 80 to 120 grit is common, but some products require different preparation.
- Rinse or clean as specified and let the hull dry thoroughly.
- Inspect exposed areas for damage, blisters and keel corrosion before covering them.
On a fibreglass hull, stop at sound primer or gelcoat rather than grinding through to glass fibres. If chemical removal is necessary, use only a stripper explicitly suitable for GRP. Silicone foul-release systems need their own removal and recoating procedure. Ordinary antifouling will not reliably adhere to them.
Repair, prime and check compatibility
Fill small, sound surface defects with an underwater-rated epoxy filler, then fair them smooth. Investigate weeping blisters or extensive damage before painting. Prepare exposed iron, steel or aluminium and prime promptly using the specified metal coating system.