Many people have asked me if it is really necessary to paint the bottom of boats used exclusively in freshwater. The answer is simple; if you leave it in the water for the season, yes, paint it. Your boat may not accumulate barnacles, but boats left in freshwater can grow a healthy beard of plant and algae slime. On the other hand, if you haul it after every use, hose it off and don’t worry about it. If you do leave your boat moored in freshwater all season, the question of what bottom paint to use is a little more complex.
There are five basic types of bottom paint:
- Soft sloughing paints
- Modified epoxies
- Thin film Teflon
Regardless of type, bottom paints today have two primary ingredients to keep plant and animal life from fouling your boat’s bottom—biocides, such as copper oxide, and what are now termed slimeocides, which act as sunscreen to keep plant and algae growth from forming. While copper oxide is the dominant ingredient in most anti-fouling paints with content as high as 80%, it’s the slimeocide that keeps the fuzz off the boat that then attracts the hard shelled animals. Think of the type of bottom paint as the delivery system for the biocides and slimeocides. In freshwater, you want strong slimeocides and harder surfaces which is why only the last two types of paint, hard ablative or thin film, are typically used.
If I had a freshwater-only boat that got left in all seasons my primary recommendation for bottom paint would be Interlux VC17M, the most popular thin-film paint with performance sailors and go fast powerboats–it has a very hard slick surface, great for performance. This paint contains both 20% cuprous oxide for a biocide and a strong slimeocide called Biolux. While it can be sprayed on for best effect, it is also easy to apply with a solvent-free roller. It dries very quickly (10-15 minutes), making it great for quick launching. The downside to this paint is that it is not compatible with aluminum boats due to its copper oxide content. Also, due to the relatively low copper content, it is not as effective in saltwater—which might not matter if you boat only in freshwater.
My secondary preferences for freshwater boats are a little bit more flexible. So called “hybrid” hard ablative paints such as Interlux Trilux 33 or Petit Vivid contain copper thiocyanate rather than cuprous oxide as the biocide. These have several advantages:
2) They’re compatible with all hull materials including aluminum hulls;
3) They come in a variety of bright colors–including white;
4) They can be used in saltwater;
5) They’re still hard enough that you can trailer your boat without worrying that the paint will end up all over the trailer bungs.
6) While not as hard and smooth as thin film paints, they can be burnished for performance.
For outdrives and props used in freshwater I recommend either Petit Alumaspray (black only) or Interlux Trilux Prop & Drive Spray (clear or 3 colors).
Regardless of paint choice, check the manufacturer’s compatibility charts, prep and application requirements, and always protect your skin, eyes and lungs.
For an overview of painting projects on all exterior surfaces — topsides, deck, and bottom — read How to Paint a Boat.