How many coats of antifouling should I apply to my boat, and what affects the number?
I lift my 32-foot sailing yacht each winter and usually roll one coat of antifouling over the old paint. Should I be applying two or three coats, and do the waterline and rudder need extra?
Answer
For a typical seasonal repaint over sound, compatible antifouling, apply two full coats, plus an extra coat where wear is heaviest, unless the product instructions say otherwise. On a freshly primed hull, one contrasting indicator coat followed by two coats in your finishing colour is a useful system, provided it meets the manufacturer's application requirements.
Check the existing paint
Establish what is on the hull and whether it is firmly attached. Extra paint cannot hold loose layers in place, and an incompatible coating may peel even if the surface looks clean.
If the existing antifouling is smooth, firmly attached and compatible with the new paint, prepare it as directed and apply the maintenance coats. If it is cracking, lifting or coming away in thick flakes, remove the unsound material before repainting. Widespread failure often means stripping back to sound primer.
- Known, compatible antifouling: follow the product's cleaning, abrasion and recoating instructions.
- Unknown paint or a different technology: check the manufacturer's compatibility guidance. A specified tie coat or removal may be necessary.
- Bare hull: apply the appropriate primer system first. Primer coats do not count as antifouling coats.
Wear gloves, eye protection and the specified respiratory protection. Contain sanding dust, scrapings and wash-water so they do not enter the yard or sea.
Finishing and indicator coats
On fresh primer, apply one coat in a strongly contrasting colour, such as blue beneath red. Once the specified overcoating interval has passed, apply two red coats of the same compatible product. Use the same manufacturer and paint type throughout.
The blue layer is an indicator. When it shows through, it reveals where the red finish has worn away and helps identify areas needing attention. Do not routinely wait until blue appears across the whole hull.
On an existing smooth hull with only a few sound layers, omit the new indicator coat and renew the finishing coats. Some products allow a single maintenance coat under particular conditions, but a thin cosmetic pass should never replace the specified paint quantity.
Pay particular attention to the waterline, bow, leading edges of the keel and rudder, and any other areas that wore early last season. An extra local coat is usually more useful than unnecessary thickness across the whole hull.
Paint, use and fouling
The quantity needed depends on how quickly the coating wears and how long it must protect the hull.