Here are some step-by-step instructions to help you achieve the best results when painting the bottom of your boat.
The best time to clean and prepare the bottom of your boat is when you haul it out at the end of the season. If you let the slime and growth dry and harden, it's much more difficult to remove. The easiest method is to have your boat yard pressure wash the bottom as soon as they haul the boat. You can also take it a step further and scrape, clean, and scrub the boat bottom after a pressure wash. Commonly, a bucket of water with Mary Kate On & Off Hull & Bottom Cleaner along with a 3M Doodlebug Cleaning Kit (scrubbing pad & holder). If your old bottom paint really starts to flake off, you should remove the old layer completely, using a scraper or a paint stripper.
Usually you can paint directly over an old layer of bottom paint, but eventually you'll want to remove all of it so you can start with a clean surface. You may also want to use a stripper when you are changing the type of bottom paint. The Back To Nature paint strippers are environmentally safe and easy to use. The trick is to lay the stuff on pretty thick and give it the appropriate time and warmth to do its thing.
If your bottom paint is in good condition, you really only need to sand it a little, clean it up, tape it off, and roll on another coat. The pros use a grinder or rotary sander with something like 80-grit paper. You can use a finish sander or a random orbital sander, they just might take a little bit longer.
Antifouling dust is a major health hazard and you really should use a dust extraction system when you are sanding. Fein and Festool both offer excellent dust extraction systems that will attach to your sander to drastically reduce airborne dust. Beyond that, you should certainly be wearing a respirator and particulate filter, eye protection, and ear protection for your own safety. In addition, wear a paint suit and gloves to keep the paint off of your skin.
The antifouling material in bottom paint is often heavy and will settle to the bottom of the can, so be sure you take the time and effort to mix your paint properly. If you have a paint shaker, run it for at least five minutes to get the copper and the pigment evenly distributed throughout the paint. If you're mixing it by hand, pour half the paint into a mixing bucket so you can mix without making a mess. Keep dredging up the copper off the bottom of the can until the bottom feels clean to the touch of your mixing stick. A drill mixer bit works best for thorough stirring. Slowly stir in what you poured off until the paint is uniform in color and consistency. Each time you add to your roller tray you will want to quickly stir again to ensure consistency.
Paint Rollers and Technique
You will want to use a semi-smooth nap (3/8 inch nap) roller cover and get an extension for the handle to give you a little extra distance from the slinging paint. We recommend not to add any thinner to your bottom paint unless the manufacturer specifies otherwise. (The exception would be a hot day in direct sun. But be careful not to add too much thinner. You are only cheating yourself of the necessary bottom paint antifouling.) Most people roll vertically up and down the hull — from the keel to the waterline. You will want to work quickly to maintain your wet edge and not let the paint dry. A second coat is usually standard, but the longevity of your bottom paint will probably benefit from a third and fourth coat. Let bottom paint dry at least overnight before you put masking tape on it to paint the bootstripe.
Generic masking tape seldom cuts a smooth line at the bootstripe. Bottom paint has a tendency to seep underneath, leaving a rough finished line. You really should use a 3M Fine Line tape for masking. If you're careful to seal it tightly against the hull, it will provide a clean, crisp line between the paint and the bootstripe.