Choosing the Right Antifouling Paint

Marine-based organisms like weeds, algae, slime, mussels and barnacles look for surfaces to attach to, and your boat's hull gives them the perfect place to settle down.

Any boat left in the water for extended periods of time faces marine growth, which creates hull-drag that affects speed and increases fuel consumption. Barnacle buildup creates pressures that can warp and damage the hull, particularly on smaller fiberglass boats.

Removing barnacles and other marine growth is an unpleasant, labor-intensive process, which is why countless boaters rely on antifouling paint. It contains a biocide that kills any marine growth that comes into contact with the hull. 

Biocides Used in Antifouling Paint

For many years, the most common biocide found in anti-fouling paint was tin. However, tin is so toxic that it was deemed harmful to the environment and was banned by the EPA in the 1980s. Since then, copper has been the most commonly used biocide in bottom paint.

Copper Biocide Anti-fouling Paints

Contain a copper biocide, either cuprous oxide or cuprous thiocyanate. Cuprous oxide is less effective and can't be used on aluminum, whereas cuprous thiocyanate is more effective at killing marine growth and can be used on aluminum surfaces.

Zinc Biocide Anti-fouling Paints

Contain zinc pyrithione, a very effective biocide against fungus and algae. It repels slime and plant buildup, but is not as effective against mussels and barnacles as copper biocides. 

The higher the copper content in a paint, the greater its toxicity and price. Although copper-based bottom paints are still used, concerns about their effect on the environment have caused some marinas to ban boats with copper-bottom paint.

Zinc paint provides solid protection against UV rays that damage fiberglass and gelcoat, and is also safe for use on aluminum hulls or sterndrive units. Although zinc-based bottom paint isn't as effective as copper-based paint, it doesn't present as much of an ecological threat as the latter does. 

Non-Metallic Biocide

Because the effects of metal-based antifouling paints on the environment are clearer, a shift toward non-metallic biocide alternatives is in effect. These copper-free biocides require lower concentrations of biocide. Paint manufacturers also offer bottom paints that contain an anti-slime agent. 

Which Biocide to Choose

There are several factors to consider when deciding which antifouling paint to use based on its biocide content. 

For example, if algae and slime are particularly troublesome in your area, a paint with zinc biocide might be preferable for your boat, whereas an area that notorious for barnacle buildup may call for a copper-based bottom paint. Local laws and regulations are also important to consider. If you moor your boat in an area that prohibits copper biocides, obviously zinc or non-metallic biocide antifouling paints are your only options. 

Types of Antifouling Paint

Ablative Antifouling Paint

Stays relatively soft even after it cures. Because it never fully hardens, the motion of water passing through it causes the outermost layer to slowly wear away, exposing fresh ablative paint underneath. The part that already released its biocide is removed, uncovering a new protective layer. 

Hard Antifouling Paint

Cures to form a solid, hard layer over the surface onto which it was painted. It's activated when it comes into contact with water. No motion or movement is required to release the biocide, which gets used up before the paint. Once the biocide is depleted, the hull needs to be repainted.

Which Type of Paint to Choose

Choosing an antifouling paint based on its biocide is never straightforward. Hard bottom paints have shorter biocide lifespans than ablatives and require more frequent reapplying. However, they're more durable and don't wash away. This makes hard paint better for speedboats or boats that run often at wide-open throttle. A big benefit of ablative paint is that it clearly indicates when the hull needs to be repainted because it wears away completely. Many boat owners choose ablative bottom paints that vary in color to the hull to easily tell when it needs repainting. Another benefit of ablative antifouling paint is that there's no buildup of old layers of paint, and you only need apply a new coat once the old one has worn off.

Epoxy and Copolymer Antifouling Paint

Epoxy Antifouling Paint

Hard paints that cure to a solid finish and slowly release their biocide. They tend to carry a higher cuprous content. Because it cures to a hard shell-like finish, epoxy antifouling paint can shrug off abrasions better than ablative paints, which makes them extremely durable.

Copolymer Antifouling Paint

Softer paints that use the ablative method. Copolymer paint is formulated to release its biocide at a steady, controlled rate, which makes it very durable. A standard ablative paint may have an expected working lifespan of 18 months, while a copolymer bottom paint may last up to three years.

Which Paint Base to Choose

Weigh up the pros and cons of each type of paint to decide which is best suited for your boat. Prolonged exposure to air can cause the biocide within a hard bottom paint to become permanently inactive, whereas copolymer paints are not affected by lengthy "dry" periods. 

So a boat that is to be stored during the winter may be better protected with copolymer paint. If the hull is commonly exposed to wear and tear such as bottoming on shallows or having lines dragged across it, a hard epoxy paint is preferable to a soft copolymer bottom paint.