Here is a Youtube link for "Fast bottom Paint Removal using Walnut Blasting".
You could also try the following steps:
There are several ways to take off many paint layers, and what you ultimately decide on will depend on your budget, the boatyard’s rules and your boat’s current condition.
Method 1:
Sanding the boat’s bottom is the dreaded chore that most boat owners
will face at some point. This is definitely a case in which having the
right tools will really make your job easier. Using a vacuum
sander helps keep most of the dust out of the air, producing a much
cleaner work environment. Many boatyards also require them. Prices for
a random orbital sander with a vacuum port range from about $60 for a
Ridgid brand to about $450 for a top-of-the-line Festool. You’ll also
need to buy a Shop-Vac-style vacuum.
You’ll want to sand down the bottom with 60- or 80-grit paper until
the excess paint is removed and the bottom isn’t overly textured. Be
careful not to damage the gelcoat. Most boatyards in the United States
have strict rules about how bottom jobs are done and may require a
ground cover or a tent around the boat from the waterline down to
capture the dust and debris.
Method 2:
A quicker way to remove many layers of paint is through
high-pressure blasting with either soda, sand, or a special glass material shot at the hull. It’s effective and faster than sanding,
but it’s also not typically a do-it-yourself project. If having your
hull blasted is something that you’re considering, do your homework
and find a reputable blaster. Also check to see if your boatyard
allows it. Expect to pay about $35 to $45 per foot for professional
blasting.
Soda blasting is what many boat owners choose for bottom-coating
removal on fiberglass boats because it’s less abrasive than other
materials, though if you have a steel or aluminum hull, you can
probably go with sand blasting. If the project is done by a skilled
blaster, the soda will take off coatings down to the gelcoat or
barrier coat—if it’s in good condition—without damage.
After soda blasting, the surface is typically too smooth for bottom
paint and must then be sanded with 60- or 80-grit sandpaper. Before
covering with new antifouling paint, wash the bottom with water and
let it thoroughly dry. According to Mike Morgan from Chesapeake
Blasting Service, “Most boats can be blasted unless they have serious
issues with the gelcoat or fiberglass. If this is the case, they
should be peeled and a whole new bottom applied.”
Method 3:
A chemical paint stripper is another way to remove paint buildup
and is the method of choice for some boatyards since there’s no dust
or noise and the residue can easily be contained. Paint stripper is
relatively easy to use—you brush it on with a paintbrush, wait the
recommended time, then remove the resulting goo with a scraper—but it
can be a time-consuming process, not to mention a messy one. A ground
cover is essential, and you’ll need to discuss the debris disposal
with your boatyard.
When choosing a chemical paint stripper, be sure to check that it can
be used on your particular hull material, and expect to use 5 to 6
gallons for a 45-foot sailboat. Paint strippers work best when not
used under direct sunlight, as the chemicals need to stay moist to
work. After the old antifouling paint is removed, you’ll still need to
sand the bottom with 60- or 80-grit sandpaper to rough up the bottom
for paint.
(Source of above points)
