Antifouling paint is also known as bottom paint, and is a necessary tool to keep the build-up of living organisms on a boat's hull beneath the water line. However, antifouling paint loses its effectiveness when there are several coats applied over the course of a few years and therefore it should be removed. There are three ways that you can remove antifouling paint: scraping it off by hand, through the use of paint-removing solvents or by sandblasting.
Have your boat removed from the water and set on boat stands by trained personnel at your boat yard. If there is any grime or barnacles on the hull, use a pressure washer to remove anything that has built up since the last antifouling painting. Allow the bottom to dry for at least six hours.
Strip the old paint using the hand scraper. Place a tarp underneath the boat to catch any paint scrapings. You will need to use a stepladder to get close to the bottom of the hull. Remove as much bottom paint as possible using the scraper. You may need to use a chisel for any paint that is difficult to remove.
Sand the remaining antifouling paint. Any bottom paint that could not be removed by scraping may have absorbed into the boat's hull. Using an orbit sander with 80-grit sandpaper, sand the remaining spots. Be sure to wear goggles, ear protection and a respirator mask to protect against debris, noise and any toxic dust. Make sure there is a shop vacuum handy to remove dust from sanding the hull.
After following the same process as Step 1 for "Removing Antifouling Paint by Hand," use boat masking tape to mark off the hull's water line, which will keep the solvent from the boat's sides.
Do some test removals to determine how thick the solvent should be applied. Pick a spot on the hull about six inches wide and six inches deep. Using a paintbrush, apply a small amount of solvent, then attach a small piece of the paint removal blanket cut to the same size. Press firmly and slowly from top to bottom, making sure there are no bubbles or gaps. After a few minutes, remove the blanket. If the antifouling paint removes completely, then you have achieved the right amount of thickness and removal time. If not, continue to test until you get the right combination.
Fill an airless paint-spraying gun with the paint-removal solvent to ensure full and even coverage. If you don't have access to a paint sprayer, you can apply the solvent with a paintbrush or roller. Be sure to wear goggles and a respirator mask. Place a tarp beneath the boat to catch any drippings. Spray all parts of the hull that has bottom paint to be removed.
After applying the paint-removal solvent, adhere a paint-removal blanket to the hull. Again, making sure no air bubbles occur. After the predetermined time (see Step 2), remove the blanket.
Complete Step 4 all the way around the hull. When finished, any remaining residue can be removed using a pressure washer.
After following the same process as Step 1 for "Removing Antifouling Paint by Hand," place a tarp underneath the boat to catch any debris sandblasting creates..
Close all of the valves on your sandblaster, then fill it with silica sand.
Pressurise the sandblaster for use. Once it is fully charged, begin removing the old antifouling paint. Spray the sand at an angle to do as little physical damage to the boat's hull as possible, especially if it is made of fibreglass. Spray the sand using large, sweeping motions.
Once you have completed sandblasting, spray the hull with a pressure washer to remove any remaining residue. Use a shop vacuum to remove any dirt or sand from beneath the boat.
Due to potential damage to the boat's gel coat, sandblasting is not always recommended, especially for a thin layer of bottom paint to be removed. There is a process called sodablasting that is similar to sandblasting, but uses a substance similar to baking soda to remove paint. Sodablasting is less harmful to a boat's hull.
Always wear eye, ear and breathing protection when working with chemicals and dust. Be sure to get permission from your boatyard before beginning any antifouling paint removal. Any damage caused during bottom-paint removal, such as by scraping or sandblasting, should be repaired before new antifouling paint is applied and the boat is relaunched. Unless you have previous experience with sandblasting, it is advisable to have the antifouling paint removed by a specialist trained in that field.