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Bottom Painting

Painting the Bottom of Your Boat


Preparation is the key:

Painting the boat's bottom with anti-fouling paint is 90 percent preparation and 10 percent application. The first step is to choose an anti-fouling paint that is the best match to the waters of your area and compatible with your hull and the existing bottom paint. Read the labels and consult with the paint manufacturers to ensure the product you choose is compatible with your boat.  If the paint you choose requires a primer use the one specified by the paint manufacturer.  Don't skip this important step.  Consult with local dealers and boaters to find the most effective paint for your waters. But remember: Get numerous opinions before making your choice.

Anti-fouling paints are toxicto both you and the environment. Even standard copper-based products should be handled with extreme care. When sanding or applying anti-fouling paint, cover your skin, hair and eyes completely, and wear an adequate respirator. Those throw-a-way paper masks do not block the volatile fumes found in bottom paint. Always follow the precautions on the label of the product you are using. Request a copy of the MSDS if you're not sure.

Inspect closely for the following before beginning:

Thick accumulations of years of paint should be removed. A thorough sanding each year will prevent such a build-up. A professional sand-blasting eliminates heavy coats more easily and with less toxic dust than by sanding. There are also chemical paint removers on the market designed specifically for removal of bottom paint. Whatever your method of removing the old paint take care that the removal of the paint does not damage the underlying hull material. Spread a plastic sheet under the boat to prevent toxic runoff that could reach the ground or adjacent water. Additionally you should ensure that the removed paint is properly disposed of. It doesn't just go in the dumpster. Allowing the removed paint dust to be washed into the water is not good for the environment we all enjoy and should protect.

Blistersrun the gamut from minor cosmetic blemishes in the gelcoat to serious structural problems. If blisters appear to be deep and are weeping fluid of any kind, or if you're getting a high reading on the moisture meter, potentially major repairs may be in order before applying new bottom paint. Moisture meters should only be used by an experienced professional as the readings are subject to interpretation. If you don't have a moisture meter, test for retained water by taping (with duct tape on all four sides) a few square feet of plastic or Saran Wrap. Droplets of water condensing inside the plastic may indicate excess moisture in the hull. Blisters should be opened, cleaned and filled prior to renewing bottom paint. If the blisters go beyond the gelcoat into the FRP laminate you should consult a Fiberglass repair professional to determine proper repair technique.

Nicks or deep scratchesin the gelcoat can be filled with a waterproof polyester filler such as America's Cup or with an epoxy filler such as Marinetex and then sanded smooth. Epoxies are more difficult to sand but provide a stronger bond to the existing hull than polyester fillers.

Inspect the keel-hull joint If movement is evident, repairs will be necessary before painting. Simply stuffing a gap with caulking or filler is no fix for the problem. It is extremely important to determine the reason the joint is loose and make proper repairs. Consult your boat yard maintenance foreman to determine proper repairs.

Hull preparation: cleaning, sanding and taping

Read the paint manufacturer's instructions carefully. This is one time you should not say to yourself "I don't need no stinking directions" All paints require the bottom to be clean--toothed or keyed by sanding--and free of peeling paint. Don’t make the mistake of sanding the hull too finely prior to painting for a smoother finish. The paint needs the toothed surface to cling to the bottom. It’s the exterior paint surface that needs to be very smooth not the underlying hull.

Make sure you clean and sand the bottom of the keel and under jack stand pads. Most yards charge extra to lift the boat or rearrange the stands, so plan your work for the minimum number of moves.

Follow the paint manufacturer's directions in cleaning the bottom of sanding dust, yard grease and fingerprints. Some paints use water; others use alcohol or a proprietary solvent. Allow the cleaner to evaporate before painting. If the manufacturer's instructions say to use a specific cleaner, us it.

Masking the hull or boot top paint gives both amateurs and professionals difficulty. Choose good quality tape such as 3M's Fine Line, since inexpensive paper tape causes many problems. Do not rush this job. Wavy or crooked paint lines detract surprisingly from the boat's appearance. Two pairs of eyes are best. One person pulls the tape and sights down the hull, while the other stands amidships calling out the straightness of the line. Taping in short lengths usually results in a choppy, angled line. Try tacking the masking tape at the bow and peeling off an entire boat's length of tape. Wrap the tape around the curve of the hull in one smooth motion, adjusting the height of the tape by eye as you go. This may take numerous attempts and adjustments. View the tape line from all angles until you are satisfied that it is straight.

Applying the anti-fouling paint:

Now it's time to paint. Methods include brushing and spraying, but most bottoms can be done by rolling on the paint. Remember to keep paint off yourself. Rubber gloves are a necessity, even to just open and stir the paint. Of course, if you are spraying, it is crucial to wear a professional-level respirator.

If rolling the paint, choose a roller cover that is made to withstand the solvents in the paint. Inexpensive paper rollers fall apart quickly. Use a thin foam roller on well-prepared racing bottoms for a smooth finish. Short-to-medium nap covers are better for rougher surfaces or for cruising boats where thicker coats of paint may be desirable.

Keep a brush handy for areas around the rudder, strut and other appendages too small to allow the roller to pass. Having a second person assisting with a brush to tip out holidays and get to those hard to reach spots will produce a high quality paint job.

Since the waterline receives the most wear, abuse and exposure to sunlight, apply extra paint to this area. Put one roller width horizontally around the waterline, then apply a full coat. With two full coats on the bottom, the waterline area now has four coats.

Pull the masking tape off soon after the last coat is finished. Pull the tape at a 4-degree angle and upward for a crisp, clean line.

Don't forget to paint under the pads and keel bottom before launching.

Make sure you didn't paint over those zincs! 

Check to make sure the seawater intake screens are not painted closed. Ream out the little holes to prevent a cooling water problem.

Consult the manufacturer's directions on drying time--some paints require immediate splash while others require drying time.

Now let's go boating.