Which Bottom Paint Should I Use?

What is Boat Bottom Paint?

Bottom paint is applied below the waterline on your boat, and typically refers to antifouling paint that prevents marine growth from clinging to your hull. Barnacles and slime can slow you down and increase your fuel costs because the engine has to work harder to move your boat through the water. A bottom full of barnacles and weeds can also put you in harm’s way because it can seriously hamper your ability to maneuver.

The key antifouling ingredient is some type of biocide for deterring hard marine growth such as barnacles and mussels. Some antifouling paints also include an algaecide for preventing soft growth like algae, slime and weeds.

There are many different antifouling coatings, making it hard to choose the paint that’s right for your boat and your budget. Some coatings work better in different locations, including fresh or salt water. Some are more environmentally friendly than others, and some cost more based on the amount and type of biocide, and the protection they provide.

Do I Need Bottom Paint on My Boat?

If you pull your boat out of the water every time you use it, you probably don’t need bottom paint. But if you keep your boat in the water all season, or if you take your boat out of the water periodically, such as on a trailer or lift, then yes, you should apply bottom paint to prevent hull fouling. Same goes whether you keep your boat in salt water or fresh water.

What are the Different Types of Antifouling Paints?

All bottom paints are not the same and will not be effective for all boats, in all waters and fouling conditions, and for all types of boating. Bottom paints are usually one of two types: hard or ablative, with variations that are effective based on how much time the boat spends in the water and how often it moves.

For example, hard bottom paints are effective for a single season, and they build up season after season, eventually requiring costly removal. Ablative paints come in single-season and multi-season varieties and don’t build up.

Ablative Bottom Paint

Ablative paint is designed to gradually wear away as water flows beneath the hull, either from use or from the movement of tides and currents. Fresh biocide is exposed as the paint layers wear away, preventing marine fouling organisms from attaching to the surface. Some ablative paint formulas are for multi-season use, and some are effective for only a single season. Also, ablative bottom paint can be copper-based or copper-free, and water-based or solvent-based. For most ablative paints to work effectively, motion is required, so this paint is best if you use your gets frequent use.

Self-Polishing Copolymer Ablative Bottom Paint

This type of antifouling paint also wears away gradually to expose fresh layers of biocide, but the self-polishing properties of its controlled-release copper copolymer formula make it work whether the boat is at anchor or underway. Some copolymer ablatives are effective for multiple seasons, and need only a light scuffing to reactivate the antifouling in the spring, just before launching.

Hybrid Copolymer Ablative Bottom Paint

Hybrid bottom paints are self-polishing and release biocides at a controlled rate like copolymer bottom paints do, but hybrid paints can be burnished–like a hard bottom paint–for smoothness and speed. Unlike hard bottom paints, hybrid paints have less buildup to remove next season.

Hard Bottom Paint

A hard bottom paint coating doesn’t wear away like an ablative coating does, it just loses its biocide over time. The biocide in hard bottom paints leaches constantly while the boat is in the water, whether the boat is moving or not. Hard paints lose their effectiveness once all the biocide leaches out. Because hard bottom paint doesn’t wear away, it builds up and has to be sanded down before reapplying bottom paint the next season. Also, hard bottom paints generally become rendered inactive one the boat is hauled out of the water, so they’re best for boats that remain in the water for extended periods. They’re also ideal for racing boats or boats operated at faster speeds because the hard paint coating can be burnished to increase smoothness and speed.

Questions to Answer Before Buying Bottom Paint

What is My Boat Made Of?

When it comes to choosing bottom paint, substrate (surface) is a big consideration. Do you have an aluminum, fiberglass, steel or wooden boat? Most bottom paints contain some type of copper biocide, which is suitable for fiberglass and wooden boats, but not for aluminum. The copper in the paint causes galvanic corrosion, which will destroy an aluminum boat or pontoon boat. For antifouling protection on aluminum boats and underwater metal parts, you have to apply a copper-free bottom paint.

Inflatable boats made of PVC, Hypalon or other materials also need bottom paint if they’re left in the water for long periods. Ablative bottom paints designed for inflatable boats won’t crack after drying or peel while you’re rolling up the inflatable for storage.

Where Do I Use or Keep My Boat? Location is Everything!

What are the fouling conditions in your area? By ‘area’ we mean not only the geographic location, but the specific harbor, and even your location in the harbor. Why? Because the fouling conditions can be that location-specific. Is the fouling light, moderate, or heavy in your part of the harbor? Typically, more biofouling occurs in waters that are warm and still, rather than colder waters where water flow is unrestricted.

Is My Boat in Fresh Water or Salt Water?

Barnacles, algae and slime are the bane of boat owners who do their boating in salt water. And even though boats left in freshwater don’t have to worry about barnacles, they can still get slimy and grow green beards. If algae slime and algae are a problem, you’ll want a paint that contains an algaecide such as zinc pyrithione (this applies to copper-free biocides, too). If you do your boating in freshwater lakes, look for bottom paints designed specifically for freshwater fouling. Why pay for extra protection you don’t need?

How Often Do I Use My Boat?

If you use your sailboat or power boat frequently, ablative bottom paint is a great choice because it’s most effective when the boat moves through the water to release fresh biocide. If you don’t use your boat as often and it spends a lot of time at the doc or on the mooring, your best bet would be a self-polishing copolymer paint that’s effective whether the boat is moving or not. In addition, brief boating seasons are suitable for a single-season antifouling paint, but for a longer boating season, it can be more effective and economical to apply a multi-season bottom paint. If you trailer your boat, a harder ablative antifouling paint that can withstand trailering and beaching might be the way to go.

Do I Want to Use My Boat for Racing?

If you want to race your boat or you just want to make your boat go faster, use a thin film bottom paint, a hard modified epoxy bottom paint, or a hybrid bottom paint that can be burnished. Burnishing involves wet sanding or using a Scotch-Brite® pad to make the surface smoother.

Should I Use Solvent-Based Bottom Paint or Water-Based Bottom Paint?

Environmental restrictions in some areas prohibit the use of solvent-based paints due to their high VOC (volatile organic compound) content. If this is the case, you can apply a water-based, low VOC bottom paint. Water-based bottom paint offers easy cleanup and is very effective against marine fouling.

A common misconception about water-based bottom paint is that it will dissolve and leave the hull with no antifouling protection once the boat is in the water. Not true. Once cured, the water in water-based bottom paint evaporates and leaves a protective film containing the biocide.

Should I Use Copper-Based or Copper-Free Bottom Paint?

Again, environmental restrictions may dictate the use of copper-free antifouling bottom paint. Typically, a higher percentage of copper in the paint means it’ll be more effective at combating shell fouling. But copper-free bottom paint containing the eco-friendly biocide Econea™ (tralopyril) has proven very effective on shell fouling such as acorn barnacles and zebra mussels. If you have an aluminum boat, you have to use a copper-free antifouling paint to avoid galvanic corrosion, which will destroy aluminum. Underwater metals also need bottom paint that’s copper free.

What Type of Bottom Paint is on My Boat Now?

If there’s bottom paint on your boat now, it’s really important that your new bottom paint is compatible with the old bottom paint. You don’t want your new paint to peel, so you need to find out what the previous paint is. If you can’t determine that, you’ll need to strip off the old paint completely and start over. Antifouling paint formulas change often, but here are some general compatibility and surface preparation guidelines. For best results, read the bottom paint manufacturer’s recommendations.

How Much Do I Want to Spend On Bottom Paint?

Typically, the higher concentration of biocide a bottom paint has, the more expensive it will be. Hard bottom paints are typically less expensive than ablative antifouling paints. Among the ablative marine paints, the single-season ablatives are typically less expensive than the multiple season ablatives.

How Much Bottom Paint Do I Need?

The amount of bottom paint varies depending on how big your boat is and how heavily you apply the bottom paint. Refer to the paint manufacturer’s details on coverage amounts (typically given in square feet per gallon). Here’s an easy formula for figuring out the square footage of your hull below the waterline, where you’ll be applying bottom paint.

Length (in feet) x Beam (in feet) x .75

Here are some typical estimates for various types of boats. These estimates assume you’re applying two coats, and that you’ll be coating the surface as thickly as possible without drips or runs.

How Often Should I Bottom Paint My Boat?

How long does boat bottom paint last? Depends on the type of bottom paint you use and your boating conditions. Typically, you should apply bottom every year, but there are some bottom paints that last multiple seasons. If you use your boat regularly or keep it in the water, check it every year to see if you need to reapply bottom paint.

Hints to Help You Choose the Right Bottom Paint for Your Boat

• Ask other boaters in your local marina – they’re a prime source of information about what works and what doesn’t in your particular location.
• If you want to paint in the fall and launch in spring without repainting, choose a multi-season self-polishing copolymer paint. This type of bottom paint has an unlimited launch window.
• All bottom paint manufacturers have a chart that shows the compatibility of their bottom paints with those of existing brands. Be sure to take a look at the compatibility chart before you buy.

TotalBoat Bottom Paint Comparison Chart

* Krypton provides full-season protection in salt water, where the water flow is unrestricted. It can provide multi-season protection in fresh water conditions.

If you have any questions about which bottom paint to use, please call our Tech Support Team at (800) 497-0010.