Meticulous preparation will always be rewarded by better results, a cleaner bottom and lower long term costs.
It can be very tempting to simply apply new antifoul over the existing paint. This is certainly the quickest way to get the boat back in the water, but it may also store up problems for the future. A good antifouling paint is not cheap – for a 40ft yacht it’s easy to spend well over £200 per coat, yet if it’s applied over rough, loose or flaking layers, then it’s almost certain to start falling off well before the season is over. In any case, once a certain number of layers have been allowed to build up, it’s almost inevitable that they will start to peel away from each other.
Small areas of loose or flaking antifoul can be removed with a paint scraper, before repainting. However if large areas are poorly adhered to the hull, then it makes sense to strip the hull back to a sound surface. The same applies once there’s a thickness of 10-15 layers of paint. Antifouling dust is extremely hazardous, so the only safe options are wet sanding, or stripping with a chemical stripper that’s formulated for use on fibreglass. Even then full protective clothing, including goggles and an effective facemask is vital – and it’s a laborious, slow task. Most owners are therefore very happy to accept the price of soda or slurry blasting. This is a much faster process so the savings in time – and mess – are nearly always worthwhile.
There’s little point investing in quality racing sails if there’s a growth of slime on the bottom of the hull.
It’s possible to delay the stage at which completely stripping the bottom paint becomes necessary – and give a smoother surface for subsequent layers – by wet sanding the hull before applying the new paint. The easiest way of doing this is to use an open mesh abrasive cloth on the end of a long pole. A couple of hours spent each season will slow the build up of paint and help to maintain a smooth finish.
Bare fiberglass should be primed with an antifouling primer appropriate to the type of antifoul used. If the hull has been stripped back to the gelcoat it’s also worth considering applying epoxy coatings to guard against osmosis. If considering this it’s important to first get a surveyor to test the moisture levels of the hull as sealing excess water in with epoxy is liable to store up problems for the future.