Found the post below in another forum. Maybe it will help answer some questions:
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Of all the methods previously described, paint stripper is probably the best. If you are trying to get to the original gel coat then it is probably the only way. I don't mean to suggest that stripper isn't messy, but sanding or blasting turns your bottom paint into tiny particulate matter which, given its toxicity, is very undesirable. With the stripper method you corral all the paint and goop, let it dry out into solids, and dispose of properly. (it is hazardous waste)
Here's my two experiences with the stuff:
I bought an '80 Montauk in 1986 from the original owner who had put bottom paint on her from the get go. There were several layers. The waterline kinda' got away from him at the bow. The paint was red. It had to go! I used a paint stripper made for bottom paint. (I don't remember the brand) It worked well in removing the paint but had the odd side effect of slightly "swelling" the gel coat. This isn't quite the right phrase, but you could tell where stripper had been appllied and where it hadn't by the fact that the gel coat was ever so slightly raised in the area where the stripper was applied. The gel coat wasn't altered in any other way in terms of its hardness or shine.
The problem I had with this boat was that the original owner did a thorough job of preparing the bottom for paint, i.e. he really roughed it up well with sand paper. Because of this I chose to repaint the bottom with white bottom paint which really looked pretty good. From a distance it did not appear to have bottom paint at all. When I repainted I corrected the waterline. I marked it in the water only one inch above actual waterline. (boat loaded and fueled). This left me with the area at the bow where the old paint had crept way up. I wet sanded up to 400 grit then rubbed it out with rubbing compound and it looked good as new.
My other experience was watching a fellow Whaler owner take to bottom paint off of his 18' Outrage. I his case, the bottom was only lightly sanded before paint was applied. As a result his bottom came out magnificently. When he got done rubbing it out, you could not tell there was ever bottom paint on her. So it can done!
A note on strippers: They are not all the same. In my contracting business here in Seattle I do a lot of old home restoration work. Most of the time we use heat guns to remove the bulk of the paint, but with hardware and detailed woodwork we use strippers. I don't think you need to necessarily use a stripper made for bottom paint. The brand I like to use most often is Jasco Premium Paint & Epoxy Remover (not to be confused with Jasco Paint Remover) It is nonflammable, water rinsible and doesn't smell to bad. It is caustic and will burn your skin if it comes in contact with it. I you get some on your skin, go rinse it off quick or you will feel it in less than a minute, (I know). The most prudent thing to do would be to buy several different products and do some trial patches on the bottom to see what works best.
