Here are some step by step instructions to help you get 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. Once you let the slime and growth dry and harden it is 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 On & Off Bottom Cleaner along with a 3M Doodlebug Pad holder will do a great job. If your old bottom paint starts really flaking off you should get out a scraper or you might want to consider using a paint stripper and removing the old layer completely.
Usually you can paint directly over an old layer of bottom paint but eventually it will all want to come off so that 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 line of strippers are easy to use and environmentally safe. The trick is to lay the stuff on pretty thick and to 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 would 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 as well as eye and ear protection for your own safety.
The antifouling in bottom paint is often heavy and will settle to the bottom of your can make sure you take the time and effort to mix your paint properly. If you have a paint shaker, run it for at least 5 minutes to get the copper and the pigment evenly distributed throughout the paint. If you are doing 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 paddle. 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. Wear sleeves and gloves to keep the paint off your skin. 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 to not 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 so that that you 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. And you will want to get hard bottom paint into the water within the time specified on the label.
Generic masking tape seldom cuts a smooth line at the boot stripe. Bottom paint has a tendency to seep underneath, leaving a rough finished line. You really should use a 3M "fine line" tape. If you are careful to seal it tightly against the hull it will provide a clean crisp line between the paint and the boot stripe.