Can Copper Antifouling Paint Be Kind?


As you’ll soon see in our upcoming bottom paint report for brackish water, cuprous oxide (copper) still rules the roost when it comes to long-term antifouling protection, with hard paints and ablative paints fairly evenly matched for durability. For those who care about reducing their impact on the ocean, this raises a question: If we want to stick with copper (as opposed to copper-free antifouling), which type of paint-hard or ablative-is easier on the environment?

With respect to bottom paints, marine biologists are most concerned about the effects of copper leaching, so it would seem a simple matter to discern which paints have the most copper in them. However, when comparing copper paints, it is important to note that the copper percentages listed on the can are usually calculated by weight. Which means that a paint that has heavier solids and resin may actually have more copper than a thinner paint that boasts a higher copper percentage. While some low-copper freshwater antifouling paints clearly have less copper than saltwater blends boasting 65-percent copper, there may only be a small difference in the volume of cuprous oxide in a paint that advertises 39-percent copper versus one that shows 45 percent.

But to really make an informed decision, it helps to understand how bottom paints work. It is not so much the quantity of copper in the paint, but the leach rate, the rate at which the copper is released into the environment that makes it effective.

When an antifouling paint goes through the EPA registration process, the maker has to prove to the EPA that it is effective. One of the ways paint makers prove this is by indicating what registered biocides are in the paint. Then, the paint maker has to provide a leach rate, which is expressed in micrograms per square centimeter per day. The scientists at EPA have enough data on some biocides that leach-rate testing is no longer required for certain biocides like cuprous oxide when submitting new registrations. In Canada, however, it is required for every paint in every color.

The key to antifouling is to establish a level at which the biocide coming out of the paint film will control fouling. Cuprous oxide is an excellent biocide for all types of fouling, but it requires a much higher leach rate to control slime and algae than it does to control barnacles. For instance, to control so-called hard macro-fouling-barnacles, mussels, worms and the like-it might take 3 micrograms per square centimeter per day of leaching copper; but for micro-fouling-slime and algae-it might take 12 micrograms.

To control both macro and micro fouling, you would make a fairly soft paint with a high load of cuprous oxide, which would be expensive and not last very long. The ideal paint will release the correct amount of biocide at the right time to last a long time. The way to do this is to formulate a paint that will control the leach rate of the biocide plus be somewhat self-cleaning so that the biocide is released at the correct rate.

After years of talk of banning copper in Washington state, it now looks like regulators are finding copper to be less offensive than some of the substitute biocides. According to some studies, paints that leach less than 9 micrograms per square centimeter per day land in the Goldilocks spot–with enough potency to prevent fouling, and not so much to overload the local waters with copper. And as it turns out, all but a few copper paints meet that leaching rate.

Can Copper Antifouling Paint Be Kind?
PS Tech Editor Drew Frye applies paint to panels deployed in Chesapeake Bay marina to test fouling in brackish waters.

This leads to a discussion of ablative paints. Ablative means “to wear away,” and there are several ways paint wears away. The most basic ablatives are soft sloughing paints that wear very quickly, with the wear-rate varying with the temperature of the water. The warmer the water, the faster it wears away.

Other paints use more complex formulas of controlled depletion polymers or controlled solubility copolymers. Generally speaking, the maker blends rosin (solids) and resin (gummy liquid) to arrive at an acceptable wear rate. Rosin is less expensive than resin, so the less expensive ablative paints will probably contain more rosin than the more expensive self-polishing copolymers that wear by means of a chemical reaction that prolongs the paint’s effectiveness. What made the now-banned TBT paints so effective was not the TBT but the tin acrylic resin (tributyltin methacrylate) that controlled the wear rate. The same is true of the copper acrylics used today.

Hard paints work by leaching biocide out of the paint film and leaving the paint film behind, which creates a honey-combed matrix surface. The biocide that is supposed to leach out is trapped in the paint film below until either the honey-combing becomes brittle enough to break off, or someone scrubs it to remove the layer of paint and exposes fresh biocide. Scrubbing of course, causes biocide to be leached into the water at a higher than usual rate, which is why some ports like the Port of San Diego have adopted bottom cleaning regulations.

A little about hard paints: Not all hard paints are rock hard. One quick way to test hardness is to rub your thumb on the paint film as the boat is pulled from the water. If there is no paint on his thumb, it scores a 10 for hardness, and if your thumb is colored with paint and you can see through to gelcoat or primer, we rate it a 1. Interlux’s VC Offshore racing paint is a 10, which means that there is certainly enough biocide being released to control barnacles until it is overwhelmed by the slime, which lets shell and barnacles gain a foothold. Popular hard paints like Interlux Ultra hover at around an 8 on our theoretical hardness scale. Ablative paints run the gamut from 1 to 8.

Some hard paints actually have higher leach rates than some ablative paints, but because of the paint film, you may never get all of the biocide into the environment. So if you got 80 percent of the cuprous oxide released from a paint that has 15-18 pounds of cuprous oxide in it, you will release 12-14 pounds of biocide. And if you get 90 percent of the cuprous oxide released from a paint that has 6-8 pounds of cuprous oxide in it-as most ablatives do-you will release only 5-7 pounds of biocide into the environment.

Comparing copper content alone is also problematic because not all of the copper leaches out of some paints. This residue copper often enters the environment as it is sanded or rinsed from your hull ashore. This brings up the issue of boatyards, a common non-point source of copper and other pollutants entering the environment. In some instances, where and how you carry out your bottom maintenance can have more impact on the environment than which copper paint you choose. Most states have clean marina programs that certify boatyards at marinas that carry out best environmental practices.

Based on published leach rates, ablative antifouling paints tend to leach out less copper over the lifetime of the coating. This is mostly due to the fact that these ablatives contain less copper than their hard counterparts and also deliver that copper more efficiently over their lifetime. However, there probably isn’t a significant difference between the softer and harder ablatives when you take into account the life of the paint. The harder ablative products, which have lower wear or erosion rates, will leach copper at a rate similar to the soft ablatives products when calculated over the longer lifetime of these harder ablatives, even though they may contain a higher percentage of copper.

Bottom line: Because of the number of variables involved, it is very difficult to make any definitive conclusion as to whether hard paints or ablative paints are the friendliest for every sailor. Total copper (cuprous oxide) content does seem to offer a good starting point, but there are many other factors to consider. If you are not interested in copper-free alternatives, but want to minimize your impact, the best approach, in our view, is to buy a paint with the least amount of cuprous oxide that meets your needs (in other words, don’t buy a two-year paint if you are going to repaint every spring). Advanced ablatives with relatively low volume of copper that releases at a controlled rate will typically fit the bill. You also want to keep hard scrubbing to a minimum, and look for a boatyard that meets your states clean marina criteria. We would advise sailors in sensitive freshwater areas to avoid soft-sloughing paints, or paints high copper content and to consider the low- or no-copper paints designated for these environments.

Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills techniques required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida.


  1. My bottom paint is 6 years old still holding up except on the gear. Wish I could recall the brand. Only thing I may be doing right is having it brushed monthly from the waterline to the tip of the 6’ keel.


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