Whether you’re rolling it on yourself or paying some marina monkey to get the job done, choosing the right bottom paint is easier said than done.
Many anglers have a love-hate relationship with their boat: they love taking it fishing, but they hate all the boat maintenance that goes along with it. And boat maintenance isn’t just time-consuming, sweat-inducing work – it can also be quite confusing figuring out what exactly needs to be done, and when. You’d think that painting the bottom, at least, would be simple. No such luck. It’s actually yet another task that requires know-how and patience. What’s worse, choosing the best bottom paint for your needs can be rather complex.
Bottom Paint Biocides
All bottom paints for boats share the same purpose: to retard marine growth. To that end, they have a biocide in them. Cuprous oxide is the most common and is generally considered among the most effective, but it’s come under fire lately for environmental reasons. (Years ago tin was the favored biocide in the form of TBT, but it was even worse environmentally-speaking and hasn’t been around for decades). As a general rule of thumb, the higher the concentration of copper in bottom paint, the more effective it is at warding off growth.
There are other, more environmentally-friendly biocides like zinc omadine, zinc pyrithione, thiocyanate, and Econea, though they just can’t match the protection level of cuprous oxide (Econia may be an exception depending on who you believe). And the latest tech includes developments like the organic compound medetomidine, which stimulates larval barnacles to keep them in swimming form rather than attach to a hull. But this remains an unproven tech, and the effect on other forms of growth are TBD.
When it comes to bottom painting aluminum hulls or drive units, cuprous oxide isn’t an option in the first place – not unless you want to set off a galvanic corrosion explosion that will eat away your boat in a season or two. More than one less-than-savvy boater has put his foot right through the bottom of an aluminum boat, a couple years after giving it a coat of bottom paint. But when it comes to fiberglass, after all is said and done, cuprous oxide remains the number-one biocide for long-term in-water storage. For those who put in their boat for a few weeks at a time and then haul out with regularity, however, these other options are worth considering.
On top of these biocides, many paints include an anti-slime agent. The biocides themselves will prevent the growth of organisms that attach to the hull, but not necessarily the growth of algae. So if you keep your boat in a body of water where a “beard” of green growth is common on boats, you’ll want to make sure you choose a paint that also includes the photosynthesis-blocker Irgarol or one of the zinc products.
Bottom Paint Types
There are two basic types of bottom paints: “hard,” (epoxy or non-sloughing) and “ablative” (which slowly wear away to expose new biocides). Then, there are also in-between hybrid paints that are semi-ablative. Which you should choose depends on how fast your boat goes, how often you use it, and whether or not you trailer it.
Hard paints hold up better to repeated and continuous high-speed use, and on boats that are pulled and trailered with regularity. The down-side is that they’re tough to remove and after a few years of application, stripping the bottom is often necessary. Ablative paints are essentially self-cleaning and once they wear away they don’t leave much buildup behind. But they’re not as tough and can easily be scraped away by trailer bunks or even by beaching the boat. Ablative paints can also lose their effectiveness if your boat sits at the dock for long periods of time without moving through the water (and thus exposing fresh biocides).
If you regularly trailer the boat or if you blast offshore at speeds of 40-mph or more, you’ll almost certainly want a hard paint. If you have a slower boat that always lives in its slip an ablative probably makes a lot more sense. And if you fall somewhere in-between, a hybrid might make sense.
Application Factors to Consider
If you’re re-painting a boat that already has bottom paint, your choices may be restricted by compatibility issues. Vinyl-based paints, for example, can only be applied over other vinyl-based paints. And hard paints can’t be applied to a bottom that’s been previously painted with an ablative paint without a complete stripping, first. (Ablatives can be painted over hard paints without an issue, as long as it’s in good shape).
There are also some paints with specific timing windows. If you need to launch the boat within a certain timeframe, be sure that the paint you choose offers a window of opportunity that works for you. Some won’t cure properly if the boat is launched too soon after painting, and others won’t cure properly if you wait too long before launching.
Choosing the Best Bottom Paint
On top of everything we’ve discussed already, some paints simply seem to work better in some climates than others. The bottom line? As you take all of the above factors into consideration, it’s also a smart move to talk with experienced professionals in your area. Boatyards that prep and paint dozens or hundreds of boats in a season, then see the results for themselves, can often offer a lot of insight into which biocides, types of paint, and brand formulations will work best in their specific areas.