Antifouling paints are surrounded by unnecessary mystery. Although the various paint formulas are complex, the concept is simple. Bottom paint is simply a mixture of a binder (a fancy term for the paint that carries the color) and a biocide (an equally fancy name for poison). The biocide kills off marine growth while the binder provides cosmetic good looks.

Binders range from the old-fashioned "oil based" paints to modern vinyls. There are special formulas for high-speed boats, sailboats and even dock queens that never move. For practical purposes, there are only two biocides, those based on copper and those based on tin.

Old-fashioned copper bronze bottom paint was little more than finely milled copper powder mixed into a soft varnish. You had to continually stir the paint as you applied it to keep the metal powder from settling out. Many formulas required that the boat be launched while the paint was still wet. This usually meant getting a crew of guys to paint the boat in the slings.

Copper bronze paints are still available, although today's formulations are a lot easier to work with. Most can be allowed to dry before the boat is launched. They are still extremely popular with wooden boat owners and with people desiring a "traditional" appearance.

Vinyl paints have been around about as long as fiberglass boats. This is probably no accident because vinyls seem to work better on fiberglass and are available in a wide range of colors. You aren't limited to the traditional bronze color. Blue, green, red, brown and even clear vinyl antifouling paints are available.

Both copper and tin-based biocides (and sometimes both) are used in vinyl paints. TBTF (tributyltin fluoride) and other organo-tin compounds, mercury, arsenic and similar biocides are no longer on the market due to environmental considerations.

Both oil-based and vinyl products are known as conventional antifouling paints. They work by letting minute quantities of the biocide leach out of the binder. The leaching process supplies enough poison to kill most marine growth until no more biocide can reach the surface of the paint.

Leaching effectively stops long before the paint begins to look dull or worn. You cannot determine the effectiveness of antifouling paint by studying its appearance. The only way you can be sure of antifouling protection is to re-paint every season.

Re-painting every year raises problems


A thick buildup of old paint will eventually become rough, causing lower top speeds and increased fuel consumption. This is the reason you see people crawling under their boats with sandpaper every spring.
Soft finish paints tend to wash away during a season of use. Less paint remains adhered to the bottom of the boat, so there is less to remove between paintings. Hard finish paints can be burnished to an extremely slick, "fast" surface for high speed applications.

A half dozen years ago we were introduced to copolymer or "multi-year" bottom paints. Unlike conventional paints, the biocide and binder are combined into a single molecule, hence the "copolymer" name. The paint is the biocide and the biocide is the paint...one in the same.

The obvious advantage is that as long as you have any copolymer paint on the bottom at all, you have antifouling protection. Multiple seasons of effective use can be obtained from this type of paint, especially in the Great Lakes region.

The original copolymer paints used either pure organo-tin biocide or a combination of copper and tin. The copper compounds were added to make the paints work better on boats that sit idle for periods of time, as most pleasure boats do during the work week. Now, all copolymer paints use copper compounds.

New is something called Compound-X from the Star Brite company. It is an additive you put in your bottom paint that, the company claims, doubles the life of the antifouling properties. Compound-X is really tetracycline hydrochloride, a relative of an antibiotic you may have taken to get well. It works by providing a bacteriostatic layer on the surface of the paint.

Don't get too excited about this new product. It is designed to prevent the growth of salt water barnacles, not the slime and weed growth of fresh water. What it will do for Great Lakes boat owners remains to be seen.

What's the right paint for your boat?


The answer depends upon a number of things. Of first consideration is the hull material. Wooden boats look best and get good protection from copper bronze paints. Fiberglass boats can use virtually any of the paints on today's market.

Aluminum boat owners can use only tin-based paints. Copper bearing antifouling paints must never be used on aluminum hulls. Copper sets up a galvanic current when submerged in water with aluminum, causing the aluminum to corrode rapidly. Painting an aluminum boat with copper antifouling paint will cause severe structural destruction and possibly the complete loss of the boat.

Generally speaking, you should never put new vinyl paint over an existing layer of oil-based paint. The solvents in the wet paint can lift the layer of the other. A similar mess can be encountered by putting a new layer of one brand over an old layer of another.

If you can't start with a clean bottom, always test a small patch of the existing paint. Be sure the new paint won't lift the old. Otherwise, you waste a lot of time and money on a paint job that will just flake or "slide" off the boat when it's launched. (Don't laugh, it happens. I've seen a boat where the new paint simply slid off the old in the water. The resulting mess discolored the hulls of nearby boats in addition to requiring an expensive repainting.)

Sanding is the best way for the average boat owner to remove old bottom paint. Be sure to wear long sleeves, gloves, eye protection and a dust mask. Avoid contact with the dust. Remember, it's full of stuff designed to kill living plants. You're not a plant, but you are alive. Wash up before you eat and never breathe the sanding dust.

A long-handled roller is usually the best tool for applying new antifouling paint. Use a 3/8-inch nap roller on a five-foot handle. You'll get a smooth finish without having to crawl around on your back under the boat.

Thin bottom paint only enough for easy application. Don't try to stretch the paint by using more thinner. Remember, when you thin this type of paint you are also reducing the amount of biocide on the hull. Less biocide means less protection. Thinning bottom paint simply wastes the money you spent on job.

Do not spray bottom paint unless you are a professional with the right equipment and a proper spray booth. The chemicals used in many of today's bottom paints require the gun operator to wear special respirators for safety. Spray painting in a boatyard inevitably gets spray dust on surrounding boats. Do you want a painted bottom or a law suit?

Standard masking tape doesn't work well with antifouling paints. The paint always creeps beneath the tape, leaving a rough line when the tape is peeled away. If you want a clean line, use the special "fine line" masking tapes on the market. (A suitable substitute is the type of strapping tape containing nylon filaments.)

Some spots on every hull require the use of a brush. Purchase a real bristle, chip brush for the job. The bristles won't be damaged by the chemicals in the paint, yet these brushes are cheap enough to throw away after one use. Don't buy foam disposable brushes as the chemicals in the paint may cause them to dissolve.

Boot stripes along the waterline are difficult to keep good looking. One solution is to protect them with wax that incorporates an organo-tin biocide. This wax prevents marine growth in the same way as bottom paint. It also helps protect unpainted fiberglass from dirt and weathering.

Dispose of all rags, rollers and paint tray liners properly. You're handling poison, so treat it with respect. Don't leave anything around for young sailors to put in their mouths.

Always keep a disposable brush and a small amount of the paint in reserve. Give both to the yard for launching day. You can't paint under the cradle pads. These spots can be reached only in the moments before launching when the boat is hanging in the slings. Most yards will apply the paint, if you supply it and a brush.