Mailport: June 2013
Bottom Paint Qs
How thick is too thick for the buildup of old layers of bottom paint? This question arises because I have just finished painting the bottom of my boat. Even though I diligently sought out potential flaking spots with my knife, while rolling on the paint (Pettit Ultima Eco), I would frequently get a mess caused by the paint flaking off. I have only owned this boat for three years, so I really do not know how many layers there are. Each year, I have applied one, sometimes two coats, using a ¼-inch nap roller. Have I prepared the surface each year inadequately, or are the previous layers just dead and thus allowing layers to flake off?
So those questions beg another: When does one strip all the old paint off, down to the barrier coat? What are the better, easier methods available to the do-it-yourselfer? Or is it more effective to have it done by a professional?
Voyager, Tashiba 40 No. 173
These are the problems commonly faced by owners of used boats with unknown bottom paint histories. We took your questions to our technical experts and Pettit Paint’s Tom Maellaro. The answers depend on the type of paint you have: hard or ablative. According to Maellaro, in a hard paint, anything around 20 mils of thickness (approximately 10 coats) will begin to lose adhesion. Having more layers on the outside of the coating will make the inner layers less flexible and more likely to 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 Once a hard paint coating has used up its antifouling properties, the product left behind is no longer active and won’t protect against growth.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.
In an ablative paint, anything around 15 mils of thickness will begin to lose adhesion—but since the coating ablates over time, it should not build up like a hard paint. Every time the product ablates, it is releasing fresh biocide, so 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. 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 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.
Without seeing the vessel, it sounds like it may be time to strip the bottom. Stripping the bottom of a 40-foot boat is a large job for a DIYer.
Your 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, March 2009 and 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 a 40-foot boat. Check out our October 2011 article, “A Mathematical Decision Maker,” which outlines a mathematical formula you can use to determine whether the DIY approach is right for you. On a boat of this size, we would recommend looking into having the hull soda-blasted (PS, October 2011).
For owners of older boats with unknown coatings, a tie coat can help with adhesion issues. 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.
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, as you found, adhesion will suffer. The rough surface will also mean more drag in the water and slower boat speed.
Next: Stripper Stresses