While 'elbow grease' is an option it is not only a desperate one but quite dangerous too. Bearing in mind the hazardous toxins you are proposing to become covered in.
It's a big one!
'Nitro-Mors', the paint stripper manufacturers even tried to market a version for this a decade or more ago. It was a right old palaver. Involving covering the whole underside with newspaper while the product 'took effect' (which it didn't anyway), and great expense and waste of time.
There are really only two ways and the first is to prevent it from building up in the first place!
Paint manufacturers, particularly for boats, love to get people plastering the stuff on as if it was going out of fashion. Fine for a really high-grade finish on your topsides and superstructure, so it won't ever need doing again if well looked after and regularly polished, but unnecessary for antifouls.
Put one coat on to begin with (two at the very outside) on a new, or freshly prepared vessel and with the exception of really specialist products such as the racing teflons, silicone coatings and copper-bearing epoxies, use a 'self-polishing' product that will remove itself as you cruise. (that's basically how these work).
Then each new season - and here's the golden rule - don't put too much on! After scrubbing at the end of the season, come the next, you only really need to apply a good band, say two-to-three feet deep all the way around, from the waterline down.
Removing to start with:
You first need a yard that has no problem with the following (some haven't got the space, and have other issues).
Next, find an abrasive cleaning operator who really knows how to do their job. This is absolutely essential. The best way is to get recommendations, even try to witness the results of work undertaken. The operators themselves will refer you to the jobs if they are any good.
And get the vessel eggshell-blasted.
There is real finesse involved in this, but there are plenty of people who do it, and some fail to appreciate this. And I've seen many superb and a few dreadful results. The thing you must strongly emphasize to the operator is that you are not having it done for osmosis repair, just for paint removal. (Think of Michael Caine in 'The Italian job, kind of!)
Blasting is no different to any other abrasive cleaning process in that there is an infinite gradation of 'cuts', from course to fine. And depending upon the material used for the blast, the pressure applied at the nozzle, and the volume of particles introduced to the blast-stream, you can either end up with a superb, clean, nicely abraded surface for new work to commence straight away (at which point, refer back to my first comments), or virtually no lay-up left on the vessel's bilges!!
So no messing about, get it eggshell blasted, and thereafter keep the build-up to a minimum.
The new(ish) epoxy antifouls are excellent. They contain copper particles and following initial application, all you do is very lightly abrade them with wet-abrasive paper each season. To expose new active ingredients. No need for repainting. Well, hardly ever - and that's got to be good.
By the way, the blast operator must ensure that the vessel is thoroughly enclosed in either a tent, or partitioned off behind fences of tarpaulin or similar, as well as carrying out the work on a windless day (having also dust-sheeted the boat), in order to minimise inconvenience to other people and their vessels, nearby - another reason for engaging the services of a real professional, not a 'cowboy'. [No offense, US readers, it's an English expression for someone who doesn't really have a clue as to what they are doing! ]
And good marinas and yards have areas specially demarcated for blasting operations, a good distance from other customers. It is a very familiar, the only method in fact, practise for repairing osmosis, where not only the paint, but some of the gelcoat and / or glass lay-up also will need removal - how deep, depending upon how bad - good operator again!!
Answered By: Girly Brains - 5/21/2010