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Painting Underwater Metal

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Painting your boat’s bottom is a straightforward job: You, or your boatyard, apply new paint over old. But what about the running gear: struts, shafts, trim tabs, and propellers? Bare metal will sport a luxurious coat of underwater flora and maybe even barnacles by season’s end, especially if you don’t use your boat often. Excessive growth produces a domino effect: It creates unnecessary drag that will impair your boat’s efficiency, resulting in poor performance and more trips to the fuel dock. In addition, it’ll cost you more to scrape off at haul out. Here’s the right way to prevent it.

Every major paint company sells special coating systems for underwater metal. Only thing is, they involve several steps and take more time and/or money than just painting the hull, especially if you’re paying the yard. Some of the better-known names are Interlux and Pettit, but just as important as the brand is the application. Read and follow the directions, and don’t cut corners. Don’t mix products from different companies, as brands use different blends of chemicals that can react violently if combined. Procedures and products differ for different metals as well. All companies generally provide detailed instructions for steel, bronze, stainless steel, aluminum, and even lead (for sailboat ballast keels) on their Web sites or in printed material available from dealers.

Whichever paint system you choose, here are the basics. First, scrape old paint, barnacles, grass residue, etc., off the metal, then degrease the surface with the appropriate thinner for the paint system you’ve chosen. Clean the metal by sandblasting it (a job best done by the yard), scrubbing it with coarse emery cloth or wire-brushing it with a bronze brush (not steel, which will rust) until the surface is bright (see photo 1). Brush off any residue, or blow it off with compressed air. Don’t use more thinner, as it will leave a residue that will adversely affect the bonding of the primer to the metal.


Apply a thin coat of primer to the now-shiny metal, leaving bare the areas that will be under the zincs (see photo 2). Let it dry for the specified time, anywhere from one to eight hours depending on product and temperature, then apply multiple coats of high-build primer to protect against corrosion (see photo 3); this will create a waterproof seal. How many coats depends on the system you’re using and the metal you’re painting, but in general you’ll need a minimum of three. If you want an extra-smooth surface, apply fairing compound after the first coat. Follow the directions for recoating intervals, as there’s a window of time during which you can safely reapply. Exceed it and you’ll have to sand before recoating.

As soon as the final coat of high-build primer is dry, follow up with the first coat of antifouling. Check the surface with your thumb: If it feels tacky and you can leave a print without getting any paint on your thumb, it’s ready. Interlux recommends two or three coats of a hard antifouling, like Fiberglass Bottomkote or Ultra for bronze and steel and one of its Trilux paints for aluminum. Pettit says to apply two or three coats of Vivid to any metal. Do it right, and next spring you can simply recoat your running gear with fresh antifouling.

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My outdrives don’t tip up all the way clear of the water, and grass and slime grow on the part that stays wet. Should I paint them?—John A., Stamford, CT

Definitely. Outdrives are easy to paint, if you spray them. The hardest job is removing all the dried gurry from last season—there are a lot of nooks and crannies to scrape before you can start painting. Every paint company has its own system, but we’ll stick with Interlux’s aerosols. (Aerosols from any of the major manufacturers will do the job equally well, such as Pettit’s Alumaspray +, an antifouling made specifically for outdrives.)

First clean the drive with Special Thinner 216, and then sand it with 80-grit sandpaper or emery cloth. Prime any bare spots with Viny-lux Primewash 353/354, then paint the whole drive with two or three coats of Primocon aerosol. Mask any areas you don’t want painted. Use the thumb test to determine when the Primocon is ready for paint, then spray on two or three coats of Trilux Prop & Drive aerosol. Of course, protective eye, skin, and respiratory gear are a must.

Of course some people are fortunate enough not to worry about painting metal outdrives. Volvo Penta Ocean Series XDP drives are composite, not aluminum, and even easier to paint. Simply wash them thoroughly with 202 Solvent Wash, apply a thin coat of No Sand Primer, and finish with Trilux Prop & Drive; use one can of aerosol per drive. Again, use the thumb test to determine when the No Sand Primer is ready for overcoating. Both metal and composite outdrives should be cleaned and painted every season.

This article originally appeared in the April 2007 issue of Power & Motoryacht magazine.