It doesn’t matter whether you’re a power boater or a sailor; applying bottom paint to keep barnacles, slime, and other marine growth off your boat’s bottom is important preventive maintenance. Sure, it’s messy and tiring work, but doing the job yourself can save you money and give you the satisfaction of knowing the job was done well. It’s a task almost any do-it-yourselfer can accomplish during the course of a weekend.

A photo of shelves full of bottom paint.

Boaters are faced with a myriad of choices when it comes to selecting bottom paint for their pride and joy. Photo by Gary Reich

Unfortunately, though, many boaters (even seasoned veterans) have lots of questions when it comes to choosing bottom paint for the season. The good news is that having a basic understanding of bottom paints and how they work can make choosing much easier.

A photo of a paint can label.

Cuprous oxide is the biocide found in most bottom paint. You can check the paint can's label to see how much is in there. Photo by Gary Reich

Copper Kills

All bottom paints contain some sort of toxic biocide that prevents critters such as barnacles from hitching a ride on your boat’s bottom. Today, cuprous oxide (copper) is the primary biocide ingredient in bottom paint, although you may also find some contain cuprous thiocyanate. Generally speaking, the more copper in the paint, the better it will work. You can look at the label of any bottom paint to find out how much biocide it contains.

Keeping Slime at Bay

Some modern bottom paints also include ingredients that inhibit slime growth, such as Interlux’s patented “Biolux” or the slimicide Irgarol. Like biocides, you can also find the amount of slimicide compounds in any bottom paint (if it has any) by looking at the label.

Like a Bar of Soap

Ablative paints are much like a bar of soap in that they slowly wash away as your boat moves through the water, slowly exposing fresh layers of biocide throughout the boating season. Look for names such as Interlux Fiberglass Bottomkote ACT or Micron CSC, or Pettit Ultima or Hydrocoat. These paints are good for boaters that use their boats often, but are not as good for folks who may not use their boat more than a handful of times a season.

A photo of a can of Interlux's Micron CSC bottom paint.

Interlux's Micron CSC is a good bet if you're looking for an ablative-type bottom paint. Photo by Gary Reich

Take a Hard Line Against Barnacles

Modified-epoxy paints are often called “hard paints,” because they continue to cure and get harder, even after the boat is launched. These paints slowly percolate a steady stream of biocide throughout the season, regardless of whether the boat is moving or not. This makes them a great choice for boats that may not leave the dock as much as others. Good choices in this category include Pettit Trinidad, or Interlux’s Ultra or Fiberglass Bottomkote.

Go Fast or Go Home

The last group of bottom paints we’ll discuss are called “hard-vinyl.” These paints dry to an extremely hard, durable finish that is tough enough to be burnished smooth, making them ideal for performance sailboats and powerboats. They’re also recommended for boats that live in and out of the water, such as those on a trailer or lift (or in a high-and-dry facility), because the hard finish resists damage from skids, floats, or trailer rollers. Interlux’s VC Offshore is popular hard-vinyl bottom paint.

A photo of a can of Pettit's Trinidad bottom paint.

Pettit's Trinidad is one of the best modified-epoxy bottom paints available on the market. Photo by Gary Reich

Now that you know the different types of bottom paints and how they work, be sure to check in with a pro at your local marine supply shop to find out what their recommendations are for paints that work well in your area. Chances are they get a lot of feedback from other local boat owners on what brands work well for them. You’ll also want to find out if the paint you’re planning on applying is compatible with what’s already there, too. Here’s hoping your upcoming boating season is slime- and barnacle-free.

Need to strip the paint off your boat's bottom? Read Bottom Stripping Basics by Doug Logan over on BoatTrader's WaterBlogged.

And for an overview of painting projects on all exterior surfaces — topsides, deck, and bottom — read How to Paint a Boat.