Sailors early on recognized the importance of keeping the bottom of their craft free of fouling, as they discovered that a hull covered with barnacles and weeds performs poorly. They used sheets of copper to protect the underbellies of their boats, and this practice is still used on some large sailing vessels.
Fortunately for modern boaters, we can now use paint with similar properties to those copper sheets. Most antifouling paints still contain large amounts of copper in the form of cuprous oxide, which is held in a binder to make it into paint. The rising cost of copper is one reason why antifouling paint is so expensive.
Unlike the copper sheets of old, antifouling paint often needs to be reapplied every year sometimes less often, depending on the area a boat is sailed in and whether it is left in the water all season.
It’s a good idea to decide what type of paint you need before focusing on cost and manufacturers. There is a confusing variety to choose from: ablative/self-polishing, traditional, hard modified epoxies, vinyl, thin film and a number of specialized varieties, each targeted at a specific application.
Ablative paints are formulated to wear away as a boat moves through the water, continually exposing fresh biocide. Because they depend on water washing away the paint’s surface layer, they are not a good choice for a boat that sits on a dock or mooring for extended periods of time. Because the rate of wear varies with water flow, leading edges that see higher velocities will require extra coats.
It’s wise to apply a first “flag” coat of paint that’s a different color than subsequent ones. When the flag coat begins to show, it’s time to repaint. Because the paint film wears away over time, minimal prep work is required before repainting.
In addition, copolymer ablatives can provide multi-season protection. Their antifouling properties do not degrade when they dry out, so these paints are perfect for trailer stored boats. All major bottom paint manufacturers produce ablatives and at least one manufacturer has a copolymer paint that does not rely on the boat’s movement to release fresh biocide, but it is sold for professional use only.
Older formulations based on rosin or rosin/resin blends, these paints vary from soft to semi-hard and generally cost the least. The softer varieties do not stand up to abrasion and are sometimes referred to as “sloughing,” which results when the biocide—almost always cuprous oxide—leaches out of the paint. These are single-season products and are an economical choice for cruising boats that are hauled each winter.
Hard modified epoxies
Based on epoxy resins, these are tough, hard, one-part paints that can be burnished in some cases. Their effectiveness and cost vary depending on the percentage of biocide used (usually cuprous oxide without additives). They can often be used to overcoat other types of antifouling, except for those that are soft or contain low-friction components. Boats usually must be launched within 24 hours of the final coat being applied, but this varies from paint to paint. These paints do lose effectiveness when out of the water for extended periods, but this does not happen immediately, so haul-outs are practical. They work by releasing biocide from within the paint over the course of the season and their effectiveness decreases as the biocide leaches out from deeper within the paint. Periodic scrubbing can offset this to some extent. Because the paint itself does not wear away it must be removed from time to time to avoid excessive buildup.
The hardest of antifouling paints, vinyls are often used on racing sail boats as they can be wet-sanded and burnished to a polished surface. They contain strong solvents, which are capable of attacking other paints, so all other types of antifouling must be removed before a vinyl paint is applied. Vinyl-painted boats must also be launched soon after application.
These are described as low-drag, high-performance paints suitable for freshwater use that incorporate lo- friction components like Teflon in addition to copper-based and chemical biocides. One such paint is stated to dry within two to six minutes and requires that a boat be launched within 10 to 30 minutes of application.
Specialized antifouling paints
Aluminum Safe: These employ copper thiocyanite in lieu of cuprous oxide to avoid galvanic attack of aluminum hulls and other underwater components. They are often available in spray cans for use on saildrive lower ends.
Boot-top: These are hard, scrubbable antifouling paints also using copper thiocyanate, but are compatible with bright colored pigments in addition to the muted colors of paints using cuprous oxide.
Transducer: Usually sold in spray cans, these antifouling paints are formulated so as to not degrade electronic performance. They may have zinc as a biocide.
Inflatable: This flexible antifouling paint is formulated to bond to non-rigid coated fabrics.
How do I choose?
Choosing an antifouling paint requires that you make several decisions: What is your existing paint and how much maintenance do you want to do? Does your boat sit on a trailer, or will it remain in the water for an extended period of time? Is the water salt or fresh? How important are boat speed and paint longevity? Since the severity of fouling varies significantly from area to area, it’s wise to consult other sailors in the area, local boatyards, and the guys behind the counter at your local chandlery.
Don’t buy on price alone; a cheap antifouling may not give you the protection you need and may instead cause headaches in terms of additional stripping and preparation. Most of the major marine paint manufacturers have websites that can be an invaluable source of information, particularly with respect to what types of antifouling can be used to overcoat other types.
It goes without saying when applying any paint: follow the manufacturer’s recommendation to the letter. Read the label carefully; the paint is expensive, and applying it to a poor surface may cause it to under perform or even fall off your hull. I’m the first to admit that on several occasions I thought I knew better than the manufacturer and ended up getting a sloppy finish. Applying antifouling paint is more straightforward than applying topside paint, but you must be careful to adhere to the application and safety recommendations.
Mask off the hull by taping along the boot-top and as necessary around depth or speed transducers. Protect yourself—a Tyvek suit is ideal—and roll on the paint with a short-nap solvent-proof roller. You’ll probably need a couple of coats. Follow and adhere to the overcoating times stated on the can. With ablative antifoulings, add an extra coat at the stem, the front of the keel(s) and the trailing edges of rudders, which tend to wear more rapidly than other parts of the boat. Pull the tape while the paint is still soft so that you get a sharp edge and use a cheap brush to coat those areas that you are unable to reach with the roller.
Getting the old stuff off
Removing years of old antifouling paint can be a real chore. Bear in mind that the paint is toxic to humans, so be sure to wear a proper respirator and a full body suit. Use a chemical stripper; they’re expensive but will save you lots of time. Wet sanding is another possibility, but it’s messy and may be illegal in your area because of the toxic waste.
Use 80-grit wet and dry paper (drywall sanding screens work well too) and use plenty of water from a hose. In most cases it is not essential to remove every last scrap of the old stuff, but you do need a smooth surface to apply the new paint to. Now is also a good time to repair any dings in the hull.
You should only dry-sand antifouling paint if you have a sander connected to a vacuum cleaner. The dust is very invasive and you will coat everything within a 50-yard radius. Also, the health risks of breathing the dust cannot be over emphasized; do everything you can to keep dust under control.
How often to reapply
Many antifouling paints have a limited life span. Even though some copolymer-ablatives will last a couple of years, most owners reapply at least one coat in the spring before relaunching if the boat has been out of the water throughout the winter. If you are in warmer water and the boat stays in all year, you should keep a close eye on its underbody; if the fouling gets harder and harder to scrub off, that’s often an indication that the antifouling paint is losing effectiveness and the time has come to recoat.
Under no circumstances should antifouling containing cuprous oxide biocide be applied to outdrive or saildrive legs. These are made of aluminum and will be subject to galvanic corrosion. Special antifouling paints using copper thiocyanite are available in spray cans for aluminum and seem to produce good results when correctly applied. There is no need to coat regular outboard legs, which spend little time in the water. Do not paint your bronze propeller, as it too will be affected by galvanic corrosion. Zinc anodes should never be painted; these are meant to be exposed and waste away throughout the season.