Bottom paint is needed for any powerboat or sailboat that stays in the water, but choosing the right bottom paint can be difficult. Here are the different types of bottom paint and some tips for doing it yourself.
Who Needs Bottom Paint?
If you leave your boat in the water for more than a week or two, marine growth will soon foul the bottom. Depending on your area this may include both soft growth like algae slime or mossy weed or hard growth like barnacles. Any growth will slow the boat and reduce fuel efficiency, and must be removed eventually - a process that is more time-consuming (or expensive) than preventing marine growth in the first place by painting below the bottom line with the appropriate bottom paint. This is usually done as part of spring maintenance before launching the boat for the season.
But with dozens of paints available from various manufacturers - typically all making great claims of effectiveness - how do you choose the best one for your boat and your own circumstances?
Types of Boat Bottom Paint
Traditionally bottom paints have contained a copper compound (cuprous oxide) that is toxic to the marine life that would otherwise grow on the hull. Some states and countries are now beginning to ban copper in paint, however, to protect the environment, although copper is still present in most bottom paints. Other biocides and technologies have begun to appear on the market, now presenting a confusing array of products to choose from.
Because bottom paints work by gradually releasing the copper or other biocide from the surface, the paint cannot last forever. Some types are intended for a single season, while others may last for multiple seasons (depending on your area, percentage of the year in the water, and other factors. Following are the most common categories of bottom paints.
- Copolymer (ablative) paint is a soft paint that very slowly wears off but is effective as long as paint remains. Multiple coats can be applied to last longer. This is the best type of bottom paint for boats that spend time out of the water, because the paint does not lose effectiveness when dry. Water-based ablatives have become available in recent years, with less odor and easier clean-up.
- Epoxy or modified epoxy paint is a hard paint often chosen for boats that will spend a year or more in the water. Unlike ablative paints it does not "rub off," so the bottom can be scrubbed or scraped to remove any growth while the boat is in the water. The hard coating remains after the copper has leached out, however, meaning that coats of paint will build up over time and eventually need to be stripped.
- Thin film paints are generally used in fresh water where marine growth is less likely to grow on their very slick Teflon surface. Because of the low friction of thin film paints, they are also favored by some racers.
- Vinyl paints are a hard type of paint that has less friction than epoxy paint (thus preferred by racers) and more effective than thin film paints in salt water. The paint surface can be polished or burnished to be very slick.
Choosing a Bottom Paint
In addition to the considerations described above, talk with other boaters and marinas in your area. Certain paints naturally work better in fresh or salt water, warmer or colder water. Take the time to learn from the experience of others in your area. Even different harbors in the same general area can vary in types of marine growth.
Don't necessarily choose the cheapest paint - which likely is a single-season paint. Depending on your boating and other factors, using a paint that lasts two seasons will likely cost less total as well as saving you time the second year.
Be sure to choose a type of paint that is compatible with bottom paint now on the boat, or remove the old paint entirely first. As a general rule, you can often apply a soft (ablative) paint over a previous hard paint (epoxy), but not a hard over a soft paint. Always check the manufacturer's instructions regarding compatibility and preparation.
Tips for Painting the Bottom
- Be sure to wear googles and an appropriate respirator during both preparation and painting. Bottom paint is toxic and may be carcinogenic. Gloves, a paint suit, and other protection are also advisable.
- Use the right tools and supplies to avoid wasting time - like a good power sander with coarse sanding discs to remove old paint. Protect the environment using the vacuum bag attached to the sander and a tarp under the boat when painting.
- Follow the manufacturer's instructions for prepping and painting. Don't thin the paint in an attempt to make it go further: thinned paint has less biocide and will be less effective.
- If the hull is dirty or has some growth on it, power-wash before sanding. Then use an 80-grit sandpaper to smooth the bottom as much as possible and give the surface some "bite" for the new paint.
- Before painting, plan your timing ahead. Different paints require different times between coats (when needed) or before launch. Some should not be applied too far ahead of launch.
- Use the recommended roller nap to apply the paint, avoiding any temptation to spread the paint too thin.
Other articles on sailboat maintenance of interest: