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So, a couple of years back, you acquired a good old boat at a pretty good price—thanks to the market—but now you’re wondering how many coats of bottom paint it has. And what kind? You’ve put on a few coats of ablative antifouling since you’ve owned the boat. It has adhered well and has done its job. But each year, the bottom looks rougher and rougher—with big recesses where paint has flaked off. You sweated out some extra prep-work this season, and thought you had a nice, durable subsurface for painting, but each pass of the roller pulls up more paint. What’s going on here?
More than likely, you probably have too much paint built up on your hull, and this is affecting adhesion. How much is too much? Well, that depends on the type of paint: hard or ablative. With a hard paint, adhesion loss will begin around 20 mils of thickness (approximately 10 coats). Having more layers built up will make the inner layers less flexible and more likely to chip, flake, and lose adhesion. Excessive mil thickness must be removed mechanically with 60-grit sanding or soda blasting. Once a hard-paint coating has used up its antifouling properties, what’s left behind is no longer active. It may still look great, but it won’t protect against growth. At most, one or two coats of hard paint is recommended. Any more than that, and you are wasting money, locking in antifouling agents that can’t do their work and bringing on the inevitable sanding or blasting job sooner than necessary.
Ablative paints will begin to lose adhesion around 15 mils of thickness—but since the coating ablates over time, it should not build up like a hard paint. As you use the boat, the paint should wear away, or ablate, and every time the product ablates, it is releasing fresh biocide. Technically, as long as an ablative coating is on the hull, it should continue to combat hard growth. That said, in our tests, we’ve found that this protection, especially against slime, diminishes as time goes on. Regular bottom cleaning can also prolong protection, but it can’t work miracles. Two to three coats of ablative paint are recommended for multi-season use, while a single coat is adequate for single-season use. An initial “signal” coat of a different color helps you know when it is time to repaint.
If you’ve been applying two coats of ablative each year for the last three years, that’s already six coats of paint for a total of 12 mils, not including the previous applications. If you aren’t using the boat often enough, those layers are building up, and a slow-moving sailboat will not ablate at the same rate as a powerboat.
If you’ve determined that you’re past either of these thresholds, you might want to consider a complete re-finish.
Paint removal options vary, and what’s best will depend on how much old paint there is and your personal preference. If there aren’t that many layers of bottom paint, you can sand them off with an 8-inch, dual-action orbital sander and 60-grit sandpaper, but this is hard work and requires careful safety precautions. Overzealous sanding can lead to dings and divots in the gelcoat.
Another option is using a chemical paint stripper like Peel Away or Franmar Soy Strip (PS, October 2011). Chemical paint strippers break down the paint’s adhesive bond on the hull and make it easier to scrape down to clean substrate that can be repainted. This is messy and noxious work, but it’s less back-breaking than sanding. We’ve found that the strippers’ performance varies by boat (and temperature), so review the articles on strippers to see which one best fits your scenario. And if you’re not happy with one product, you might want to try another before giving up.
Both of these methods will be time-consuming on larger boats. Before tackling this project, check out our October 2011 article, “A Mathematical Decision Maker,” which outlines a formula for determining whether the DIY approach is right for you. On boats in the 40-foot range (or larger), we recommend looking into soda-blasting the hull (PS, October 2011). If you've got an old barrier coat that needs renewing, and are still considering a non-blasting regimen, then you'll first want to read technical editor Ralph Naranjo's account of his barrier coat removal project.
For owners of older boats with unknown, well-adhered coatings, a tie coat can help make sure successive coatings stick. The major bottom paint manufacturers—Interlux, Pettit, and Sea Hawk—all have priming/tie-coat products. One that we’ve recently used with success is Sea Hawk’s 1277 Barrier Coat Primer, a chlorinated rubber compound. Pettit’s version is called Tie Coat Primer 6627. Interlux markets its Primocon for this purpose.
How long you wait to strip down the old paint depends on how bad the adhesion issue is and your tolerance for a rough bottom; work boats go years without stripping. Eventually though, adhesion will suffer. The rough surface will also mean more drag in the water and slower boat speed. If you do start fresh, regular application of an ablative paint—one to two coats for a single season, no more than three for long-term cruising—can prevent paint build up. For help in selecting an ablative bottom paint that is best suited for where you sail, check out our March 2013 report on favorite bottom paints by region.