Bottom paints have changed so much in the past few years that you may find the choice difficult when it comes time to freshen up the hull.
Antifouling paints have different purposes and characteristics, and the choice for a wood or fiberglass hull often comes down to how you use the boat. Where you use the boat also matters, because marine organisms vary geographically. Paints that perform well in one location may fare much worse 20 miles up the same coast.
The best you can do is to read up on antifouling paint, talk to your local yard foreman (he can tell you what people with similar boats are using) and have some knowledge of what is already on your hull. Armed with that information, you should be able to narrow the choices.
BOTTOM PAINT BASICS: The magic of antifouling paint is that it contains biocides, agents such as copper that kill off marine growth. Much has been written about the impact of these paints on the marine environment. But the truth is that, except for very expensive silicone paint systems like Interlux Intersleek, used primarily by commercial vessels, almost every bottom paint available is still a copper paint.
Copper is a strong deterrent to barnacles and other marine growth, and it remains legal to use in U.S and most foreign waters. The only other substance commonly used in garden-variety antifouling paints was TBT (tri-butyl-tin), and that is now banned by most maritime nations.
Today, we’re left with two choices in copper paints: water permeable, also known as leaching, and impermeable, also called ablative or self-polishing. These are available in four types of finishes: sloughing (pronounced “sluffing”) soft-resin paints; vinyl-based sandable hard-resin finishes; modified-epoxy finishes; and newer water-based paints.
These paints contain copper or copper oxide powder, which is dispersed in an agent that cures by combining with oxygen in the air. Solvents are added to make it fluid enough for painting, but these solvents evaporate before the paint cures. The curing occurs while the paint still feels tacky, which is why it is important to follow the manufacturer’s instructions for the amount of time you wait between coats. If the second coat is put on too soon, the first coat never cures and may not adhere perfectly.
Almost all paint systems available on the market, except some of the vinyls, are modifications of this basic, oil-based resin vehicle. These products are described as alkyd, oil, modified-epoxy or hard-epoxy finishes, with no distinct dividing lines between types.
Even the more recent water-based paints still use an oil-based resin as the vehicle. The water is merely the solvent with droplets of the oil-based resin suspended within. After application, the water solvent evaporates, the resin coalesces and the paint cures. Once cured, the water-based paints are just as waterproof as oil based paints. The great difference is that they provide easier clean-up and emit fewer volatile organic compounds.
SURFACE PREPARATION AND APPLICATION: Use caution when changing bottom paint systems. If the new paint is not compatible with the old, you’ll have to strip all the paint down to the bare hull before starting.
Depending on the paint you buy, preparation and application may vary and it is important to read and follow manufacturer’s instructions. Each paint is different in its chemistry and in how it reacts with the hull, the water and marine organisms. Therefore, while most can be applied with a brush, each paint does have a unique method of application that needs to be followed for optimum results.
For example, Interlux Fiberglass Bottom Coat and Interlux Super Bottom Coats are properly applied over an epoxy bottom coat. This is done by giving the prior bottom paint a pressure wash, degreasing with Solvent Wash 202 and sanding with coarse- to medium-grit emery cloth. Next the sanding residue is removed, the hull is primed with one coat of Interprotect 200E, it is faired smooth with Watertite filler, re-primed with four more coats and then the bottom coat is laid on.
For Trilux II, however, the process is slightly different. The hull is pressure washed, degreased and sanded, but the priming is done with Primocon before applying three coats of Trilux II.
Read the manufacturer’s literature and follow the instructions, including the recommendations for drying times. Painting is sweaty work, and you want to see optimum results.
I’ll make some specific recommendations on paint systems for different situations in my next post.
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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.
