OK, here's Antifouling paint 101. Hard bottom paints are considered to be single season paints and, if they are cuprous oxide based biocides, they will have a chemical change that takes place when the boat is removed from the water and the hull dries. Therefore, they are not recommended for trailerable sail boats that go in and come out several times over the season. Single season, hard bottom paints work because the biocide leaches away from the paint keeping the hull clean. It leaves the paint behind. So, if you look at an older application and see paint, you cannot necessarily assume that there is any toxicants left. If you pull the boat after a reasonably fresh application, so that you know that there are toxicants present, you cannot just relaunch the boat after the hull dried, because of that chemical change that I wrote about. If the application is recent, then you can burnish the hull to open up the pores (so to speak -which more work people don't want to do). Since the biocide leaches away from the paint, eventually you will get a build up of paint, which will get thick and at some point you will want to take this all off and start over.
Now, ablative paints are different in nature. First of all, you can pull the boat and let the hull dry and it will have no adverse affect on antifouling performance. So, it is considered great paint for trailer sailors for this reason. But, it is great for boats that remain in the water as well. Secondly, both the paint and the toxicant leech away at the same time so that you do not eventually get a build up of paint like you do with a hard bottom paint which has to someday be removed (saves work).
Ablative paints are considered to be multi season paints and not single season paints. The recommended application for single season paints is two coats. 3 coats are recommended for ablative paints. As mentioned, if you see a single season paint application where paint is visible on a hard finish, you cannot assume there is any toxicant left. But, with an ablative paint, if you see paint, you can assume you still have antifouling protection because the paint and the toxicant leech at the same rate.
Think of an ablative paint as a time capsule cold pill. The medicine in a cold pill has different capsules that kick off at different rates so that you have cold remedies over a longer period of time. Well, with ablative paints, since the paint and the toxicants leech at the same rate, all the time you have paint wearing away you have more biocide exposing itself keeping your hull clean. What this means is you don't have to have as high as a percentage of toxicants to do the job. Trinidad was always lauded because it had the highest concentration of biocide, but it still was a hard paint and didn't have the same attributes as the multi season ablative types. Micron technology from Interlux, for example, is all about this time capsule approach and it works very well. When you see that the Micron type paint is wearing thin, then you know it is time to apply more paint. Micron technology is recent technology. Hard bottom paints are old technology and you will see them disappear in time. Also, Trinidad like paints are for salt water and not freshwater use because you don't need to have that sort of high percentage of toxicants for fresh water use. Interlux' product similar to Trinidad is Ultra.
