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You are here: Home » DIY » Hull & Deck Repair » Bottom Coating Antifouling Paint

Bottom Coating Antifouling Paint

Brimstone (sulfur) was an early antifoulant. Copper came into vogue about 250 years ago, nailed to boat bottoms in sheets. Today, cuprous oxide or copper thiocyanate are blended into paint to protect boats against barnacles, tube worms, mussels and other invertebrates that can’t tolerate the copper. Herbicides like Irgarol are often added to combat algae and slime in growth-prone harbors. Econea, originally used to protect wood from insects, is the newest barnacle poison — likely the future of bottom paint given the current trend in environmental regulation.

Calling it “poison” is a misnomer. “You’re not going to find a pile of dead barnacles under your boat,” says Jim Seidel, Interlux’s assistant marketing manager ( Biocides simply prevent growth from adhering to the hull. The biggest variable is the technology used in the paint to keep a fresh layer of biocide at that micro-thin interface between boat and sea. Ablative paints wear away like soap, exposing new paint and fresh copper each time the boat moves or is cleaned. With “hard” coatings the paint stays in place. It’s the copper in the paint that oxidizes away, replenished by fresh copper deeper in the paint film.

Ablative paints require much less copper per gallon than hard paints do for the same antifouling properties, but this protection comes with caveats. On boats that often cruise over 30 knots, many ablative paints wear too quickly. Boats that sit unused for prolonged periods don’t adequately wear away ablative paint, so there is no fresh copper to inhibit growth.

“The terms ‘ablative’ and ‘hard’ are starting to get blurred because of new technology,” says John Ludgate, president of Pettit Paints ( Hybrids such as Pettit Vivid or Interlux Fiberglass Bottomkote NT (for new technology) “polish” more slowly than previous ablatives did. Others, like Interlux Micron 66, react chemically with salt in the water for precisely controlled release, whether the boat is used or not.

What type of biocide and paint you select depends largely on where and how you boat. An active sportfisher often cruising at 35 knots might go with hard paint, while a 9-knot expedition yacht is a definite candidate for a soft ablative. A boat that’s hauled every winter, painted every spring and used in an area with minimal growth should do fine with inexpensive paint, while other areas require all the fouling protection possible. Factors that encourage growth include water temperature, sunlight, nutrients from runoff or estuaries, and currents that bring fresh supplies of barnacle larvae and nutrients.

Seidel says. “If you’re sanding [between coats] it’s going to last a good long time, but if you’re just power washing, you’re hanging new paint on top of all those layers of old.” The result after a few years is fresh paint coming off in large flakes, firmly attached to old paint that no longer holds. Common solutions include blasting the bottom with baking soda or glass beads to remove paint without harming gelcoat. Viking and other yards favor The Farrow System, which uses light volcanic rock with hot, high-pressure water ( Most yards will guarantee their bottom jobs for 3 years. To do this they take all the old paint off and apply an epoxy primer then the bottom paint while the primer is still tacky.

For these applications TUFFSTUFF An epoxy high build barrier coat by Sea Hawk Paints is a good choice. It is a 1:1 mixing ratio and super high quality. Can be applied on fiberglass or metal.

With proper paint and prep as well as frequent use or regular cleaning to keep the bottom fresh, most boats can get 18 months out of a bottom job and 10 years or more before stripping old paint. For hard or ablative paint, the rule is one coat per boating season.

Metal boats and running gear have an additional problem of galvanic reaction between cuprous oxide and aluminum or bronze. Epoxy barrier coat isolates metal from copper in paint. Still, many choose paint like Interlux Trilux 33, Boero SuperNavi HA 642 or Pettit Vivid with copper thiocyanate, which is less reactive than cuprous oxide. It’s also white, allowing brighter bottom-paint colors than cuprous oxide’s dark, rusty red.

Many paints can’t be removed from the water once wet: They form a coating of oxidized copper that won’t leach away yet won’t prevent growth. If you like to sit on the hard for hurricanes or repairs, pick a paint that can be reactivated with a good pressure wash — most ablatives and hybrids can. Similarly, paints like Micron 66 that react to the salt in seawater don’t do well after more than a few days in fresh water. Know what paint is on your bottom before hauling.

While yards like to plan on a short turnaround to get customers in quickly, bottom paint needs to cure before the boat is launched. “There’s touch-dry, through-dry and full cure,” Seidel says. “You’ve got to allow that paint to reach full cure.”

Q: How much anti-fouling bottom paint do I need?

A: To determine the amount of bottom paint needed for your boat:
Total Surface Area = Length x Beam x 0.85 x 1 coat Gallons Required Per Coat = Total Surface Area/300 sq-ft

Links to More info

Protect your Bottom with Epoxy Bottom Coating
By Roger Marshall • Posted: Oct 31, 2012