Too Many Layers of Bottom Paint?

Posted by at 01:43PM - Comments: (7)

Sea Hawk's chlorinated rubber primer 1277 is applied to Skimmer, a Cape Dory 25.

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 Once a hard-paint coating has used up its antifouling properties, what’s left behind is no longer active.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.

Comments (7)

What kind of paint was used over the Sea Hawk 1277 Bottom Paint Primer as a finish coat?
I have a 17 ft pontoon boat used only in a fresh water lake w/ a electric motor.
Thanks for help.

Posted by: husstommy | January 14, 2019 2:44 PM    Report this comment

No one has mentioned paint scrapers. Although they do require some physical strength they make less dust than sanding, (use at least a dust mask and throwaway suit) the chips can be more manageable than sanding dust, (use clear plastic or plastic tarps) and you can sometimes remove a lot in a relatively short time, even if you have to finish or work on stubborn areas with a sander

Posted by: Erps | April 20, 2017 5:39 PM    Report this comment

Last year I removed VC 17 from my Sabre 362. It was a bear of a job.

In some areas the paint came off quickly and easily, in other areas not so much. Around leading edges and areas where the paint had worn off, a few minutes with 40 grit and 6 inch random orbit sander did the job. In other areas the copper had built up and was hard as a rock. The build up was like a sheet of copper that was exceptionally hard and not particularly responsive to sanding.

Tried a couple chemical removers and solvents, they did little to nothing to remove the paint, mostly just smeared it around.

Posted by: DGL | April 20, 2017 12:27 PM    Report this comment

An Eco-quip Vapor blasting system by Graco is very clean, controlled & quick. Auto paint shops may have or know who has the system in your area. The system uses high pressure water which is vaporized with tiny glass beads. A tarp below the area collects the glass beads for recycling. No chemicals, no dust. I saw this system in action by Painter George Custom Coatings in Northport, FL. Impressive!

Posted by: 630 | April 20, 2017 9:01 AM    Report this comment

Did the 28 year removal of old bottom paint on my Mirage 32 last year. Used Soy strip and then sanded with a festool random orbit sander attached to a festool vacume dust collector. Finished with 4 coats of Interprotect 2000 and a couple coats of Micron CSC. Took me over 4 weeks, working 2 or 3 hrs a day. Big "once in a lifetime" job to tackle but the smooth bottom made it all worthwhile. The festool gear was worth every penny and saved a few years of my life I'm sure.

Gerry Connolly

Posted by: Gerry C | May 15, 2013 3:30 PM    Report this comment

VC-17 remains popular in the Great Lakes and other midwest lakes. My understanding is that successive coats of VC-17 dissolve previous coats and a uniform thickness (thinness) is always present. This assumes, of course, that the VC-17 is applies over bare gel coat or, perhaps, a good barrier coat.

Is this a correct assumption?


Posted by: Thomas L W | May 15, 2013 1:34 PM    Report this comment

Don't know if this observation was scientific or not, but when I pulled my 1963 Columbia, Sine Metu, out of the water I decided it was time to remove the (then) 47 years of ancient bottom paint as the paint chips were a nickel thick. I went with the Soda Blasting method, which removed the multiple, multiple layers of safely down to the original gel coat. As this was also my test to see if a rebuild would be worth it, I needed to see the condition of the hull -- and I'm happy to report that there wasn't a single blister to be found! With three layers of epoxy barrier coat, and two layers of anti fouling my little 50 year old, 24' sailboat swims like new!

s/v Sine Metu

Posted by: JBWrites - | May 15, 2013 10:55 AM    Report this comment

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