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 found to be highly damaging to the marine environment and was banned by the Environmental Protection Agency in the 1980s. Since then, copper has been the most commonly used biocide in bottom paint.

Copper Biocide

Antifouling paints containing a copper biocide either use cuprous oxide or cuprous thiocyanate. Cuprous oxide is less effective as a biocide and cannot be used on aluminum, whereas cuprous thiocyanate paint is more effective at killing marine growth and can be painted on aluminum surfaces.

The quantity of copper within the paint is a direct indicator of its toxicity: the higher the copper content, the higher the toxicity, and the more expensive the paint. Although copper-based bottom paints are still widely used, concerns about copper's effect on the environment are increasing, and some marinas don't allow boats with copper bottom paint.

Zinc Biocide

Zinc-based antifouling paints use zinc pyrithione as the biocide. Zinc pyrithione is very effective against fungus and algae, so it will repel slime and plant buildup, but is not as effective against mussels and barnacles as copper paints. 

Painted boat hull

Zinc paint provides solid protection against UV rays that can 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. As of this writing, there are no known prohibitions against boats protected by zinc antifouling paint.

Non-Metallic Biocide

Ultima Eco antifouling paint

Because the effects of metal-based antifouling paints on the marine environment have become clearer, a shift toward non-metallic biocide alternatives is happening. These new copper-free biocides are being called the future of antifouling paint. They require much lower concentrations of biocide, and include products such as Interlux Pacifica Plus and Pettit Ultima Eco.

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 particularly effective on hulls in areas that are notorious for slime buildup. Interlux Micron Extra is a good example of a bottom paint that contains 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. 

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 is notorious for constant 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 most effective way to go, you should consider other antifouling paints with lower price tags to suit your budget.

barnacle marine growth

Types of Antifouling Paint

There are two types of antifouling paint: ablative bottom paint and hard bottom paint.

Ablative Antifouling Paint

Pettit ablative bottom paintAblative bottom paint remains relatively soft, even after it cures. Because ablative paint never fully hardens, the motion of water passing through it causes the outermost layer to slowly wear away, exposing fresh ablative paint underneath. In this way, the portion of the paint that has already released its biocide is removed, uncovering a new layer of paint that still contains protective biocide. Ablative paint continues to release biocide and prevent marine growth until it is completely worn away, at which point the hull will need repainting.

Hard Antifouling Paint

Pettit unepoxy hard antifouling paint

Hard bottom paint is a traditional style paint that 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 continues to be steadily released as long as it remains in contact with water — until the biocide within the paint is fully depleted. In effect, it is the biocide that is getting used up, not the paint. Once the biocide within the antifouling paint has been depleted, it will be necessary to repaint the hull.

Which Type of Paint to Choose

Once again, it is never straightforward when choosing which type of antifouling paint to use based on its biocide. 

marine growth boat hull

Hard bottom paints tend to have a shorter biocide lifespan than ablatives and require more frequent reapplying, but they're more durable and don't wash away. This makes hard paint much better for speedboats or boats that spend much of their time 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 is no buildup of old layers of paint, and you only need apply a new coat once the previous one has worn off.

An important consideration when choosing bottom paint is the surface it will be applied to. An ablative paint can be painted over a hard one, but hard paint shouldn't be applied to ablative paint, as the ablative will wear away, taking the hard paint with it. In other words, the existing paint on a hull will have a big influence on the new bottom paint you'll have to use. Always carefully read the paint manufacturers' recommendations and instructions regarding a specific paint before purchasing it.

Epoxy and Copolymer Antifouling Paint

In general, bottom paint will either be epoxy-based, or polymer- and copolymer- based.

Epoxy Antifouling Paint

Pettit anti-fouling paint Trinidad 75

Epoxy bottom paints are the hard paints that cure to a solid finish and slowly release their biocide. Epoxies can carry a higher cuprous content, with an epoxy such as Pettit Trinidad 75 consisting of 75% copper.

Because it cures to a hard shell-like finish, epoxy antifouling paint can shrug off scratches and abrasions better than ablative paints, which makes epoxies 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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