The Lowdown On Bottom Paint

By Frank Lanier

While death and taxes may be better known, there's a third certainty in life many boat owners face that's often viewed with just as much trepidation.

Photo of man painting the bottom of a boat
Photo: Frank Lanier

Ever since our first troglodytic ancestor shoved off astride his trusty log, mariners have tried most every concoction imaginable to keep waterborne critters and growth at bay, from mixtures of tar, sulfur, and brimstone, to paints laced with tin, arsenic, pesticides, and even the occasional jar of chili powder. Although environmental concerns have relegated the most noxious mixtures of yesteryear to the dustbin of history, bottom paint still remains a complex topic. Here's a look at what bottom paint is, its application, and how to find the one that's right for you.

What Is Bottom Paint, Anyway?

Bottom paint (aka antifouling paint) is a paint or coating designed to discourage weeds, barnacles, and other aquatic organisms from attaching themselves to (and in the case of wooden boats, eating) the underwater portion of your boat's hull. Bottom paints have traditionally accomplished this by inclusion of a biocide, with copper being the most commonly used today. In general, the more copper or other biocide a paint contains, the more effective (and costly) it is. Copper replaced tin, the previous biocide of choice for decades, now banned in most marine applications due to the environmental damage it caused while leaching into the waters of the world. While copper may still be king, the latest generation of eco-labeled bottom paints utilize little or no copper, a response to the increased scrutiny copper-based paints have received as potential environmental pollutants.

Bottom Paint — Who Needs It?

If your boat stays in the water year round or during most or all of the boating season, application of bottom paint is pretty much the rule. For boats left in the water for short periods of time (that two-week summer vacation, for example) or that are stored out of the water (trailer, lift, dry storage facility), a good coat of wax and regular cleaning are typically all that's needed to maintain a clean hull.

Tip icon A 3- to 4-foot extendable roller handle will save your back and make painting much easier.

If your boat has never been bottom painted and you're mulling over the idea, make sure you understand that once bottom paint is applied, the die is cast, and that periodic bottom-paint application, cleaning, and renewal become a permanent part of the vessel's routine maintenance schedule. Another consideration is that the application of bottom paint in some cases (particularly on trailersized boats) may be viewed as a negative to potential buyers if you decide to sell later on.

What's Your Type?

Despite seemingly endless choices, traditional bottom paints can be divided into three broad groups: ablative, hard, and hybrid paints.

Ablative (aka self-polishing) paints gradually wear away a tiny bit at a time to reveal fresh biocide as your boat moves through the water (think a bar of soap that wears away each time it's used). One advantage to this is that as long as paint remains on your hull, you know it's working to prevent growth (like that soap bar, which may get smaller, yet still lathers up). Another benefit is that as it's constantly wearing away, there's no buildup of old paint, which can be a pain to remove when the time comes. But ablative paints can be less effective if your boat remains idle for extended periods of time, which denies it that self-cleaning action provided by water movement. The flipside to this self-cleaning feature is that ablatives aren't a good choice for fast-moving boats, which would accelerate the ablation process and cause rapid paint loss.

With hard bottom (aka non-sloughing or hard modified epoxy) paints, it's the copper biocide that gradually wears away (rather than the paint itself), allowing the water to penetrate deeper and deeper into the paint until all biocide is depleted. Hard-bottom paints form a tough, hard coating that holds up well and doesn't wear away, making them a good choice for faster boats. They can even be burnished, allowing racers to squeeze every possible bit of speed from their hull. On the downside, when the copper is depleted, the hard, tough coating remains and it can be messy and difficult to remove, particularly if multiple layers of paint have been added over the years. Eventually this accumulated paint reaches critical mass, becoming so thick it begins to crack and peel, necessitating a complete stripping of the hull.

Photo of painting boat bottom with small roller
Photo: Billy Black
Bottom paint is a "must have" for boats that spend most of the year in the water.

Hybrid or semi-hard ablative paints bridge the gap between ablative and hard paints by incorporating the benefits of both into a single product. They provide a hard, smooth surface that resists buildup yet holds up to faster speeds and repeated haul-outs without loss of protection.

The type of boat you have plays a significant role in bottom-paint selection. Highspeed powerboats will want to use a hard or possibly hybrid bottom paint — ablative paints simply won't last as long at speed. Trailerable boats are best off without bottom paint, but if that's not an option, consider a hybrid paint, which will hold up better throughout the constant launch and haul-out cycles (most hard paints can't tolerate being out of the water for extended lengths of time). Slower craft can use hard, hybrid, or ablative type paints, but the benefits of the latter two (no buildup, self-cleaning, etc.) should be considered when choosing. (For an indepth review of bottom paints and long-term test results, see Practical Sailor magazine:

Application Considerations

For boats not currently painted, application is simply a matter of following the paint manufacturer's instructions for application to a new or unpainted hull. If your boat is currently painted, you have a few considerations to address before repainting. Not all bottom paint types are compatible, so unless you plan on removing ALL of the existing bottom paint, the paint you currently have will influence your choice of new paint. If you know the brand/type of bottom paint on your boat now, the options are straightforward — either continue to use the same paint or (if using a different paint) consult manufacturer specs for both paints (old and new) to ensure compatibility.

If you don't know who the manufacturer of your current paint is and plan on painting over it, at a minimum you'll need to determine whether it's a hard, semi-hard, or ablative type. One straightforward way to do this is by rubbing the existing bottom paint with a wet rag. If the paint readily rubs off, it's most likely ablative. The general rule is that ablative paints can be applied over properly prepared hard or semi-hard paints, but hard or semi-hard paints can't be applied over ablative paints (which wouldn't provide a proper adhesive surface for the hard paint). For those wanting to switch from soft to hard paint, the ablative paint would need to be removed first.

Depending on the paint and its condition (i.e. good adhesion, no flaking, etc.), proper surface preparation of your old paint can range from a light sanding to complete removal or application of a primer coat. New paint adhesion will only be as good as the paint beneath it, meaning paint in poor condition or showing signs of significant adhesion failure must be removed. Err on the side of removing more paint rather than less, but beware: If your boat has a barrier coat applied, make sure your methodof bottom paint removal doesn't damage or remove it along with the old paint.

Choosing A Paint

Tip icon Alternating ablative bottom paint colors (first coat red, second coat blue, for example) gives a visual cue as to when your paint needs renewing.

As you can tell by the sheer number of bottom paints on the market, no one type or brand works best in all water, weather, and locations. A boat moored in Maryland will require a different antifouling paint from one cruising tropical waters. One of the best sources of information on which paints work best for a particular area is your local boatyard manager. Fellow boat owners can also provide valuable local knowledge regarding which paints work best in your location. Finally, many bottom paint manufacturers such as Interlux ( and Pettit ( offer online charts and tables to assist in choosing the right antifouling paint for you boat and location.

While there are plenty of bottom paints to choose from, there's no magic bullet that stops growth altogether. Boats kept in the water year round or for the season will still require regular cleaning and periodic hauling for repainting. In northern climes where boats are hauled annually, bottom jobs are usually rolled in with the other spring launch preparation rituals. Bottom job schedules for other locations vary, but are typically required every two to three years.End of story marker

Frank Lanier is a marine surveyor with over 30 years of experience in the marine and diving industry. He holds a 100GT master's license, and has captained and maintained many different types of vessels.

Published: April/May 2014


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DIY Or Yard?

The only allure of doing your own bottom job (cleaning, prepping, and applying antifouling paint) is saving money. It's a messy, tedious job that isn't fun by any stretch of the imagination, but it's not rocket science, either. Owners comfortable with DIY projects can do their own bottom job with satisfactory results, as long as they follow the manufacturer's instructions and rules regarding containment of old bottom paint during removal (as well as proper disposal afterward). Many bottom paints are toxic and have to be treated as such; some even require a pesticide license or other special permit to apply. As such, any potential DIYers will need to ensure they're thoroughly familiar with the entire application process and follow all required safety precautions and practices, such as the use of respirators, protective clothing, etc. In addition, many boatyards have their own rules about boaters taking on this chore.

Paying a professional to tackle the job has several advantages. The time and effort required for a bottom job can (and likely will) be greater than many newcomers estimate, particularly if the hull is in need of substantial prep work prior to painting (i.e. removal of old paint buildup, blister repairs, etc.). The possibility of this "unexpected" extra time should be factored in when weighing the savings of a DIY approach versus the yard, as should the ancillary costs of painting equipment and safety gear.


Rejuvenating Hard or Semi-Hard Bottom Paints

Refer to manufacturer instructions for your specific paint, but the following are general guidelines for launching and relaunching of vessels with hard or semi-hard (modified) paints applied.

  • Newly painted boats — Launch delays of up to 60 days after painting with no loss of performance or special requirements necessary
  • Boats launched two to 12 months after painting — Scuff-sand with abrasive pad or 220-grit sandpaper prior to launch
  • Boats launched more than 12 months after painting — Light sanding with 100-grit sandpaper followed by recoating


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