Which bottom paint is best for you depends on two things: how you use your boat and whether you keep it in the water when it is laid up.

I'm going to make some recommendations that I hope will help, but they remain general in nature because the best paint for you will depend in part on where you keep your boat. Marine growth differs from place to place and a little local knowledge goes a long way when selecting bottom paint.

I'm basing my recommendations on personal knowledge and experience with Interlux paint systems, but over the years I have used Woolsey, Pettit, Awlgrip and West Marine paints, all of which have similar products that yielded similar results. My current mahogany cruiser has a red "Trinidad" modified-epoxy bottom paint manufactured by Pettit.

CRUISERS: For large cruising motor yachts in cold climates, I suggest old fashioned soft sloughing paints, which are cheaper and usually renewed each spring before launch. The least expensive, which last about one season, are the most economical, but are also the most labor intensive.

In warmer climates most cruising power vessels stay in the water year around. Consequently, they are better suited to a modified epoxy that is formulated to last several seasons. Such paints will have a slower biocide release, with enough copper for two or three years. Examples are Woolsey Neptune, Pettit Trinidad or Trinidad SR, Interlux Fiberglass Bottom Coat or Super Fiberglass Bottom Coat. Of course, these cost more than single-season paints.

Cruising sailboats can generally do the same. If hauled out for winter storage for longer than two months, then the annual application of a sloughing paint is best. If kept in the water, then a multi-year modified epoxy may work better.

TRAILBERABLE BOATS: Trailerable sport-fishing and power cruisers are subject to scraping on trailer rollers and repeated pressure washings, so they shouldn't use the less expensive paints. Sloughing paints are too soft, and modified epoxy leaching paints may lose their effectiveness if left out of the water for several couple of months. Consequently, vinyl-based paints, which provide a smooth and sandable finish, work best.

Vinyl paints are hard, tough and difficult to remove, which makes them ideal for trailerable boats. But be cautious: if you have already used other paints, you cannot switch to vinyl without stripping down to the bare hull. The solvents in the vinyl are so strong they will lift most other paints. Examples are VC Offshore (Teflon) and Woolsey Vinelast. If your local government restricts emission of volatile organic compounds, as they do in California and Michigan, you may want to use one of the newer water-based hard leaching paints, like Woolsey Hydrocoat (Teflon).

Trailerable racing and cruising sailboats might try the same, or Interlux Baltoplate Racing Finish. If you plan to put your boat in the water only for a 90 to 120 day racing season, and still want some anti-fouling capability without losing your hard racing finish, look into Interlux VC-17m, a thin-coat, Teflon and copper "speed skin" paint, which has limited anti-fouling capabilities.

ALUMINUM HULLS: The choices in bottom paints for aluminum hulls are dependent upon the boat's prior paint history, and its compatibility with the new paint system under consideration. This is because copper is not compatible with an aluminum hull.

As I mentioned in yesterday's post, most bottom paints contain copper. However, copper is more noble than aluminum, and the aluminum loses electrons to the copper when the boat is immersed in salt water. This means the paint can corrode the hull unless it is properly isolated with primer and barrier coatings.

Generally speaking, the problems are the same no matter what kind of aluminum the boat is made of, because both 5052 series used for inland boats and the 5086 series found on ocean boats have the same corrosion resistance characteristics.

To find out if the prior paint on your boat is compatible with the new paint you would like to use, investigate your hull's history by contacting the previous owners. They may have paint receipts from the last haul out, which can tell you what is on the hull. Another idea is to find out where they had it painted and ask the yard which paint systems it was using. If that fails, then you need to scrape off the bottom paint and see what is underneath, or have it evaluated by an expert.

For many years we enjoyed anti-fouling paints specially formulated for aluminum and steel hulls, which used TBT (tri-butyl-tin), a wonderful biocide that killed all marine growth and lasted from four to eight years between paint jobs. But it also killed all the marine growth in the marina and the surrounding waterways, so it is now permanently banned on recreational boats by most maritime nations. Although, TBT is still approved for use in U.S. waters on larger commercial vessels over 24 meters (81 feet) long, we are left with only two choices for aluminum hulls.

The best choice is a modified epoxy, co-polymer incorporating cuprous-oxide. This is long lasting and has the best anti-fouling characteristics, but it requires a new hull or one that has been sand blasted down to bare metal. Then a proper epoxy barrier coating is applied, followed by a paint such as Interlux Fiberglass Bottom Coat or Interlux Super Bottom Coat. These contain enough copper to last two or three years without renewal. Going this route – a barrier coat followed by a copper paint – also requires a well constructed and functioning zinc galvanic protection system.

The second choice, which is the only economical choice for use with a previously-painted boat, is a soft sloughing paint like Trilux II, which uses a Copper Thiocyanate that is more compatible with aluminum. However, it only lasts about a year, and must be renewed annually.

For more information on aluminum painting systems read chapter 14 of Boatbuilding with Aluminum by Stephen F. Pollard and the Interlux Boat Painting Guide.

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Capt. Alan Hugenot is a naval architect and marine surveyor based in San Francisco, where he operates an 81-foot motor yacht converted from a Navy patrol boat. His column appears here weekly.