Bottom Paint (Antifouling): Do You Need It, Which Type, and How to Repaint

Slime and growth come off easily while still wet but harden like concrete once dry. Most yards pressure wash at haul-out and scrape hard growth. Ask for a second pass if loose paint is flaking under the spray. Use the yard's wash pad so the runoff is captured.

Pick at edges where paint is lifting with a putty knife. If it peels off in sheets in many places, the old paint has lost adhesion and needs to be stripped, not painted over. New paint only sticks as well as the layer under it. A thick, cracked, "alligator skin" buildup also means it is time to strip.

• Old paint sound and the same type you will use: scuff sand, clean, and recoat.
• Flaking, very thick, or switching to an incompatible type (for example to vinyl): strip. A sharp hook scraper is the safest, least messy method for most owners. If you use a chemical stripper, it must be one made for fiberglass; ordinary strippers can attack gelcoat. Large strip jobs, soda blasting and barrier coats are usually best left to a yard.

Lay tarps under and around the work area and weigh them down. Wear your respirator, goggles, gloves, coveralls and earplugs before you start sanding. Keep kids, pets and neighbors' boats clear of the dust.

Use a random orbit sander hooked to a shop vacuum with 80-grit paper. Many yards no longer allow sanding without a vacuum. Do not use a belt sander (it gouges the hull), and avoid a grinder unless you are experienced: it can cut through paint and into the fiberglass in seconds. Sand off most of the last layer rather than piling coat on coat. If you see a different-colored "flag" coat underneath, that tells you when to stop.

A bottom that was never painted still has mold-release wax on it. Clean it with a dewaxing solvent and plenty of clean rags before sanding, otherwise sanding drives the wax into the scratches. Then follow the paint maker's system for bare gelcoat, which may include a primer or an epoxy barrier coat.

Look for blisters, cracks, soft spots and damage around through-hulls, the keel joint and the transducer. Check the anodes and replace any that are about half gone. Fix problems now (see our hull blisters and gelcoat repair guides), because you will paint over them next.

Wipe off sanding dust with the cleaner listed on the paint can. Run fine-line tape along the top edge of the bottom paint. Do not paint anodes, their mounting spots, the propeller shaft, or transducer faces unless the transducer maker allows a specific coating.

Buy enough first. Interlux's rough estimate of the area to paint is length overall × beam × 0.85. Divide that area by the coverage printed on the can to get the amount for one coat, then multiply by the number of coats, plus a little extra for the waterline and the spots under the stands. Interlux also has an online paint estimator. Running out halfway means a second batch that may not match.

The heavy biocide settles at the bottom of the can. Use the yard's paint shaker or a drill paddle and keep going until nothing is left on the bottom of the can and the color is even. Don't thin unless the label says so. Stir again every time you refill the tray.

Roll from the waterline down toward the keel using a short-nap roller on an extension handle. Use a brush for tight spots around struts, intakes and the keel. Work fast; many bottom paints dry quickly. Wear sleeves and gloves: this paint does not belong on your skin.

Check the recoat time on the can; many paints can be recoated by the time you get back around the hull. A second coat extends the life of nearly any bottom paint, and ablatives often benefit from a third. Many owners add an extra coat at the waterline, bow and leading edges where paint wears fastest. Keep some paint for the spots under the stands.

Once the rest is dry, have the yard move the stands one at a time (never move them yourself). Sand and paint the bare patches. Touch up the bottom of the keel when the boat is in the slings before launch.

Most bottom paints list a minimum dry time before launch, and hard paints also list a maximum time out of the water. Plan your launch date before you paint. Let the paint dry overnight before you tape over it for a boot stripe.

Bag the tarps, chips, dust, used rollers, tape and rags and take them to the yard's hazardous waste collection or your local household hazardous waste site. Never pour leftover paint or cleanup solvent down a drain, onto the ground or into the water.

If you hire a diver, use one who follows local in-water cleaning rules and uses soft methods; aggressive scrubbing removes paint and puts copper in the water. Note the paint brand, type, color and date in BoatMend so next time you can stay compatible.