By Tom Burden
If your boat stays in the water at least part of the year, good antifouling paint is critical for keeping it performing its best, and for reducing fuel costs. Most boaters find bottom painting messy and tedious, but it’s one of the key preventative maintenance jobs that keep your boat in shape. A clean hull is safe, fast and efficient, and a fouled bottom cuts your top speed, damages maneuverability and lowers your fuel economy.
Antifouling paint keeps marine organisms, taking the forms of shell (animal fouling from barnacles and zebra mussels), weed (plant growth) and slime (single-celled algae) from attaching themselves to your boat. Most antifouling paints use the dissipation of metal as a toxic biocide (on the hull’s surface and in the water) to prevent these nasty critters from adhering. Attaching copper below the waterline was one of the first effective strategies; the hulls of wood sailing ships from centuries ago were sheathed with solid copper. Today, boaters apply underwater antifouling paint that is up to 70% copper, and cuprous oxide is the most popular active bottom paint biocide.
Choosing an antifouling paint is regional, as boaters in the Great Lakes, Pacific Coast, Southeast, Gulf Coast and other regions tend to choose similarly to their neighbors in the local marina. Your choice also depends on how you use your boat and the type of paint you applied in the past (since there can be compatibility problems between some types and others). In general, softer paints can be applied over harder paints but not vice-versa. There are exceptions however, so it’s always best to consult a compatibility chart.
Use paint with zinc or copper thiocyanate biocides. Bottom paints containing cuprous oxide can’t be used on aluminum because of galvanic action. Vivid, Trilux 33, with cuprous thiocyanate biocide, and zinc pyrithione-based Trilux 33 Aerosol and Alumaspray Plus are formulated for use on aluminum. Conventional paints will destroy an aluminum hull in short order.
Use paint with copper thiocyanate, zinc or ECONEA biocide. White copper (cuprous thiocyanate) is clean white in color and used in Pettit’s Vivid and Interlux Trilux 33. It requires 50% less content than the heavy, dark copper used in conventional antifouling paint. Produces the brightest colors, whitest whites and blackest blacks. Also, unlike most metal-based antifouling agents, ECONEA can be used to easily formulate light and bright paints, resulting in brilliant colors with better consistency. ECONEA does not cause discoloration in the presence of sulfides, as do metal-based paints.
Use a paint with zinc or ECONEA biocides.
Non-biocide paints—foul release coatings: Biocide-free foul release coatings are just beginning to be available to recreational boaters, used on propellers in products like the popular PropSpeed. They’re like super-ablative paints, but with no copper biocide. These super-slick paints, such as Intersleek 900, work well on the hulls of fast commercial vessels that get used every day, but they’re finicky to apply, requiring specialized application by specially-trained painters.
Use paint with a slime-reducing additive like Irgarol or Biolux. These additives block photosynthesis near the water’s surface and restrict the growth of algae.
Use copolymer ablative type paint. Copolymer paints release biocide at a constant controlled rate throughout their lives, wearing away or “ablating” much like a bar of soap. Paint wears off faster in higher drag areas on the hull and appendages. These paints work well in high-growth areas and continue to be effective after haulout at the end of the season. In the spring, the paint is reactivated with a scrubbing or light sanding and you’re ready for another season. This is a huge time-saver for those living in northern climates. While the percentage of copper in the paint is important for evaluating its effectiveness, copper content is not the only consideration. Controlled polishing, the technology that controls how quickly the paint wears away, is another way we measure the effectiveness in a copolymer ablative paint.
We recommend a covering of two or three coats on the first application. Copolymer paints with anti-slime additives are best for heavy fouling areas. Environmentally preferable: CFA Eco, Ultima ECO and Pacifica Plus are ECONEA-based copolymer ablatives.
Ablative paints (of the non-polymer type) work in a similar way and minimize the annual ritual of sanding when applying a fresh coat. Best use is as a single-season paint for boats that are used often, but are not serviced by a diver. Not recommended when you want a super-smooth bottom finish and have a diver maintain it, since scrubbing removes paint and reduces longevity.
Make sure you remove the old paint film unless it’s also vinyl based.
There are specific paints that are recommended for freshwater, and some paints that are specifically recommended against freshwater use.
Frequently used boats may want to use an ablative paint, which will get smoother over time and will shed light growth. Infrequently used boats may want to use a modified epoxy paint that will have good antifouling properties when the boat is inactive.
If you keep your boat in the water year round you are most likely a candidate for a high-copper-content modified epoxy paint that prevents growth by leaching biocides upon contact with water. Contact leaching paint releases the biocide at a steadily decreasing rate, leaving the hard coating of the original thickness at season’s end. Higher copper content, rather than the type of paint binder as with ablative paints, generally means greater effective performance in this paint type. Modified epoxy paints adhere tenaciously to most surfaces, and can be applied over most types of paints. Since these types of paints do not wear away, a flakey layer-cake of buildup will occur with each new coat and eventually your hull will need to have the coating stripped.
Use a copolymer paint that remains effective for indefinite periods of time. Copolymer paints, such as West Marine PCA Gold, Interlux Micron Extra and Pettit Hydrocoat, offer true multi-season protection, lasting as long as there is a reasonable coating thickness. Because they expose new biocide until the coating is worn completely away, additional coats add to their longevity
Three general rules:
For more information on for the do-it-yourself boat owner on how to prep your boat’s bottom and apply antifouling paint, see our West Advisor and companion video called Do-It-Yourself: Bottom Painting. To check for compatibility of your choice of new paint with the existing paint on your boat, see our Bottom Paint Compatibility Chart (provided courtesy of the paint chemists at Pettit Paints).