How to Paint a Boat Bottom

It's a dirty job, but you have to do it.

Many of us enjoy working on our boat, doing any number of different jobs. Bottom painting is not one of the enjoyable jobs. It’s the bane of do-it-yourselfers, one of the dirtiest, most uncomfortable, least-looked-forward-to tasks of the year. And yet, bottom paint we must.

CHOOSE A BOTTOM PAINT
Getting the right paint isn’t always as easy as it sounds, because there’s a wide range of antifouling paints on the market and some are better than others for specific a climate or region. We were prepping a boat for a summer in the brackish waters of Chesapeake Bay, which is home to a number of fouling organisms. But we also needed a paint that could withstand occasional trailering, and that could be applied well in advance of launching (some paints require the boat to be launched in a specific time frame after application). Petit (pettitpaint.com) Hydrocoat Ablative Antifouling Paint (1) was ideal for our needs. Before choosing a paint for yourself, however, do some homework and figure out which is best for your personal needs.

PREP & SAND
Like with any paint job, preparation is key. If the boat’s bottom hasn’t been painted before, there’s typically an extra step required to prep the surface. In the case of Hydrocoat, we had to first treat the bottom with 92 Bio-Blue Hull Surface Prep, but the exact process can differ by paint product, so be sure to follow the manufacturer’s recommendations.

Assuming your boat isn’t being painted for the very first time, the critical task is to remove any loose or flaking old paint. If you paint right over the old stuff, the new paint won’t last long. The good news is that sanding (2) is an effective way to remove loose paint, and the entire surface will need to be sanded anyway prior to painting. The bad news is that the entire surface will need to be sanded prior to painting, which is not a fun job, but it can be made easier with an electric sander, especially on large boats. A couple of warnings:

• Always wear a dust mask when sanding.
• Lay out a drop-cloth to catch any loose flakes and bits of old bottom paint, and dispose of them properly.

An older boat with layer upon layer of old paint will have to be stripped. This is a separate job in and of itself, which makes the entire process even more onerous, and may take several attempts.

After the entire surface has been sanded and any loose paint removed, eliminate the layer of dust that now covers the bottom. Wet a rag with 120 Brushing Thinner and thoroughly wipe the bottom. Be sure to rotate or change the rag constantly as it becomes clogged with sanding dust. If you’re not 100 percent sure you removed all the dust, wipe the boat’s bottom a second time.

TAPE THE WATERLINE
If the boat doesn’t have a previous paint line, you should launch it and leave it in the water for about a week, to establish a waterline. Use painter’s tape to strike a tape line along the upper edge of where you’d like the paint to stop. Raising it at least an inch above the waterline is usually a good idea, especially if your boat is kept in an area where dirty waterlines and staining can be a problem.

Also, be sure to tape over anything you don’t want to be painted. Fishfinder transducers, for example, should not be covered in regular bottom paint, because it can negatively affect their performance. Outdrives and motor mounts may also need to be left unpainted, or be painted with specialized paint. Warning: Aluminum drive units and hulls can react with some bottom paint and set up an electrolytic reaction that can lead to severe damage. Before painting any aluminum, be sure the chosen paint is compatible.

APPLY THE PAINT
Be sure the paint is thoroughly mixed. Remember that many paints, including the Petit Hydrocoat we used, contain cuprous oxide, which tends to settle. A lot of stirring can work, but it’s best to have the can of paint shaken on an agitator to ensure it gets thoroughly mixed. Then stir it again by hand right before using it.

With the tape in place and the paint thoroughly mixed, now’s the time to fill the tray, roll the roller through the paint until it’s saturated and begin rolling it onto the bottom. Move slowly, so the roller doesn’t throw paint in every direction. At the tape line, roll up over the edge of the tape to ensure complete coverage (3). After the bulk of the bottom has been covered, use a brush to touch up difficult-to-roll areas (4), such as around the drive unit or fishfinder transducer.

FINAL TOUCHES
Before the paint dries, peel back and remove all of the tape (5). After the paint has dried and cured completely, shift the boat on the trailer to cover areas that were obscured by trailer bunks or rollers. Then it’s time to launch the boat and enjoy yourself — or at least move on to other DIY projects.

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