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.

boat antifouling paint

Any boat left in the water for extended periods of time faces marine growth. Not only does marine growth buildup create hull-drag that affects speed and increases fuel consumption, but 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 (or bottom paint). It contains a poisonous biocide that slowly leeches out of the paint into the water, killing any marine growth that comes into contact with the hull. 

Marine growth buildup bottom paint

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 cause some marinas to ban boats with copper-bottom paint.

Painted boat hull

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 new copper-free biocides require much lower concentrations of biocide. Paint manufacturers also offer bottom paints that contain an anti-slime agent in addition to the main copper, zinc, or non-metallic biocide. These paints are effective on hulls in areas notorious for slime buildup.

Which Biocide to Choose

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

marine growth algae on a boat

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. Price is also an important consideration. Although a bottom paint with a high copper content might seem like the best way to go, consider other antifouling paints with lower price tags to fit your budget.

barnacle marine growth

Types of Antifouling Paint

Ablative Antifouling Paint

This 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. As such, the portion of the paint that already released its biocide is removed, uncovering a new layer that still contains protective biocide. 

Hard Antifouling Paint

This 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. 

marine growth boat hull

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.

Consider the surface when choosing bottom paint. An ablative paint can be painted over a hard surface, but hard paint shouldn't be applied to ablative paint, as the ablative wears off, taking the hard paint with it. In other words, the existing paint on a hull has a big influence on the new bottom paint you'll have to use. Carefully read the manufacturers' recommendations and instructions regarding a specific paint before purchasing it.

Epoxy and Copolymer Antifouling Paint

Epoxy Antifouling Paint

Epoxy bottom paints are the 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

Copolymer bottom paints are 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

As always, you'll need to weigh up the pros and cons of each type of paint to decide which will be best suited for your boat.

painted boat hull

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 your hull is commonly exposed to wear and tear such as bottoming on shallows or having lines dragged across it, a hard epoxy paint will be preferable to a soft copolymer bottom paint.

 

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