Bottoms Up: Tips for Your Next Bottom Painting Project

Is a bottom job coming up on your list of boat projects? Here are some DIY tips from George Day and Tony Knowles.

Many production boats have cast iron keels, which have the bad habit of rusting once water has migrated behind the usual epoxy barrier coat applied by the builders. This was certainly the case on the Jeanneau 45.2 that we bought. After a winter on the hard, we decided it was time to make some fundamental repairs.

The “industrial” approach would be to put the boat in a secure shed, sandblast off all old coatings, then recoat it with epoxy and new bottom paint. But this would be costly and our keel was not that bad. Instead we opted to try to kill as much as we could with patches, and then apply a new barrier coat.

With the boat out of the water, we used a wire brush wheel on an electric drill to remove any loose paint, rust and scale. The now shiny exposed iron was immediately treated with Ospho (phosphoric acid), which creates a chemical coating that inhibits rust. Once we had all of the rust spots ground down, cleaned and coated with Ospho—which needs an hour to dry—we were ready for the next phase.

We decided to go with the Interlux system, so we started by wiping down the whole keel with Interlux 216 solvent, which removed all of the dust, grease and dampness from the keel. The next step was to coat the whole keel—not just the areas we had treated with Ospho—with Watertight Epoxy Filler to create an impervious barrier that would prevent water from getting to the iron. Using a four-inch plastic rectangular trowel, we applied a thin but thorough coat of epoxy and tried to keep the build-up of filler as smooth as possible. The filler went on easily, covered well and cured reasonably quickly with no sags. Once the epoxy had set, we hit the surface with a sander to fair high spots and trowel edges. To ensure that the epoxy thoroughly covered the keel, we repeated the process with a second coat and a second round of fairing.

The keel was now ready for Interlux 2000E epoxy barrier coat primer. This two-part product on top of the epoxy filler provides a hard, impermeable coat that adheres well to the epoxy filler coat and is a good primed surface for antifouling paint. You should note that you have to let 2000E’s two parts “formulate” for 20 minutes or longer before applying to the keel. It goes on like fairly thin primer paint. To get as good a barrier coat as possible, we rolled on eight coats of the 2000E.

Now, for the final coat. The boat was going to be in the water for about 16 months before the next haul out and would be in New England for the summer, down the ICW during the fall, and then in the Bahamas for the winter before sailing home the next spring. With that much use and variation in water temperature, we wanted the best antifouling and anti-slime paint available. We settled on Interlux Micron 66. We rolled on two coats overall and two more, hand brushed, along the top three feet of bottom below the waterline, where sunlight tends to hasten growth.

Now, eight months later, the boat is free of fouling and the keel is showing no signs of rust. Oddly, the bright red of the Micron 66 stays red under the water, but the paint above the water at the waterline has faded to pink. When we haul this fall, we will probably have to touch up a few rust spots on the keel, but we estimate that we have reduced the problem by at least 90 percent. And we think the 66 is as good an antifouling paint for our purposes as any on the market.

Tony Knowles is the principal at Newport Marine Surveyors in Middletown, Rhode Island and is noted as one of country’s leading yacht surveyors. He can be contacted at

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