Maximizing Bottom Paint Coverage

Posted by Darrell Nicholson at 06:09PM - Comments: (12)

Dan Spurr
Dan Spurr

We were interested to compare coverage using different types of rollers. The usual 3/8-inch nap cover is on the left. The sausage roller, good for squeezing into tight spaces is on the right.

We had a special visitor down to Sarasota, Florida this week, former Practical Sailor editor Dan Spurr was launching his newly purchased boat, a 1977 Pearson 365 named August West that he and wife Andra are hoping to enjoy in their retirement. Along with being the longest-running editor of Practical Sailor (11 years), Spurr is the author of several books, including Spurr's Guide to Upgrading the Cruising Sailboat, Heart of Glass, a fascinating account of the early days of fiberglass boatbuilding, and Your First Sailboat, a buying guide to 84 of the more popular used sailboats on the market. He is also an editor at large at Professional Boatbuilder.

Not wanting to miss the opportunity to put Spurr back to work, we enlisted his help in testing Jamestown Distributors’ bottom paint Total Boat. Introduced fairly recently, Total Boat is manufactured by Bluewater Paints, a perennial contender in our bottom paint test. Dan applied two Total Boat paints to the hull of August West. One was Spartan, a multi-season high-copper (45 percent by weight) ablative paint. The other was Underdog, an economical low-copper (24 percent by weight) ablative paint. Ablative paints, as the name suggests, gradually wear away over time.

Branding its own bottom paint is a smart move for Jamestown. The Rhode Island-based company specializes in boat maintenance products. It has set itself apart from other online retailers with kits that supply boaters with all they need to carry out common projects like waxing a hull or painting a bottom. For its bottom-painting kits, the company supplies a roller cover with a 3/8-inch nap, which is the most commonly recommended size for bottom painting. (The kit also includes paint trays, disposable gloves, thinner, and Tyvek suits for protection.)

The type of roller use is relevant because it can affect coverage, depending upon how it is used. Theoretical paint coverage claims can vary among paints, but most conventional anti-fouling (as opposed to thin-film paints like Interlux VC-17) claim between 300 and 400 square feet of coverage per gallon when roller applied. Following these estimates one should be able to meet the manufacturer’s recommended thickness, usually about 2 mils dry, or 3-4 mils wet. Most boaters don’t take the time to measure mils using a thickness gauge, they usually just apply the recommended number of coats. (The recommended number of coats for Spartan is 2-3 coats.)

So for most of us, the question remains. Are we laying down coats that are too thick? For the year-round sailor, buying and applying too much ablative paint isn't a big deal (although, after three coats adhesion can be problematic). The protection should last longer. But for those who haul out every year and repaint, putting too much paint on the hull or paying top dollar for an expensive multi-season paint is a waste of money.

In my own experience, I’ve found that I rarely can match the manufacturer’s coverage estimates using a 3/8-inch nap roller cover, so for the August West project we decided to do a comparison. Granted, this was not a proper test, just something to fill the time on a windy afternoon when the paint was drying faster than expected. Being a spontaneous notion, we used whatever rollers we had laying around the shop.

One section of the boat was painted with a foam, solvent-resistant hot-dog or sausage roller cover, a technique that, though tedious, has worked for me in the past. Larger diameter solvent-resistant foam rollers which would work faster, are also available (see comments below). The sausage roller, more commonly used for topside finishes, soaks up far less paint than a conventional, 3/8-inch cover.

The adjacent section was painted using a conventional 3/8-inch nap roller. A proper test would be to paint larger sections, or even identical hulls, with each of the rollers—but not having time, nor another Pearson 365 at our disposal, we forged ahead. (This topic will be revisited in greater detail in a future report.)

Obviously, coverage was only part of the equation. We also wanted to see how well each application performed. We will be checking back to see how the paints are doing, and hopefully get some telling photographs when August West is hauled out again.

On two designated test patches, Spurr applied eight ounces of paint with each roller cover, after accounting for pre-soak. The results were striking. Using the conventional 3/8-inch nap, the 8 ounces of paint covered 7 square feet; using the foam sausage-style roller, he was able to apply nearly 17.5 square feet—2.5 times more area. Multiplied out, this yields per gallon coverage (one coat only) of 112 square feet with the 3/8-nap roller and 280 square feet with the sausage roller. Neither comes close to applying the 400 square feet that the manufacturer says we should expect, which suggests that even the thinly applied paint from the sausage roller is too thick. (Perhaps Spurr was applying too much pressure?) Or, more likely, the makers’ estimates are overly optimistic.

Unfortunately, we did not check mil thickness of each application (this will be a part of our future report), but ultimately what matters is how well each approach performs against barnacles. We assume the area painted with the 3/8-inch nap roller will offer better protection because it has more biocide, but it will be interesting to see the results. Spurr will be hauling out for hurricane season, so we should get a good look in about four months. 

The experiment raises a question, is the 3/8-inch nap roller, the ideal choice? Certainly for rough hulls, the thicker nap does cover divots and dings that a shorter nap might miss. And it also requires far less work. But bottom paint is expensive—an ounce can cost more than a gallon of gas—so we hate to see it go to waste. If you have some tips for stretching bottom paint, I'd love to hear them. Comment here, or send me an email at

[Editor's note: This post was edited on 1.20.2016 for clarification, based on comments made prior to that date.]

Comments (12)

Thanks for your comments.
This isn't a test. It is a conversation starter, as I put it: "something to fill the time on a windy afternoon when the paint was drying faster than expected." Tests are generally available only to subscribers, with exceptions for archival material and critical safety reports which I have made available to everyone.
I appreciate everyone taking the time to comment on this topic, and look forward to more helpful input, advice based on your own experience, or raise a question that we could address in future antifouling paint tests.
Although only subscribers have access to most of our tests, all sailors benefit from the work of the dedicated people behind this organization, and we recognize that a dialogue is important. Non-subscribers can stay abreast of upcoming work and comment on it through this blog and our Waypoints newsletter, which are free to everyone.

--- Darrell Nicholson, Editor

Posted by: sailordn | January 30, 2016 12:20 PM    Report this comment

I cannot believe you guys published this. I really hope you do not go about your other tests in this manner. My seventh grade science teacher would have failed me if I presented this, OMG!

Posted by: rpjn59 | January 29, 2016 1:22 PM    Report this comment

Read the article carefully before you post. " Spurr applied eight ounces of paint with each roller cover, AFTER accounting for pre-soak."
Let people wonder about your intelligence, open your mouth and remove all doubt.

Posted by: Mhuss | January 21, 2016 7:20 AM    Report this comment

Oh for heaven's sake.

Anyone can get more or less coverage with any roller by simply rolling out thinner or heavier.

Everything else equal, the roller nap dictates load and texture not WFT.

For ablative paint, the thicker the DFT, the longer the paint can wear, before requiring recoat (to a point). For any ablative paint, one should apply a reveal colour coat first (e.g. blue) and then a desired colour coat (e.g. black). We apply a second coat on high wear points, leading edge of boat, rudder, keel etc. and roller width at the water line. On recoat, only the areas where the reveal colour shows through are recoated. This prevents build-up and failure.

Lastly, watch the washer pressure and proximity on haul-out. Those not in the know will wash excessive amounts of ablative paint off.

Rod Brandon
Marine Service Provider

Posted by: Oh for | January 20, 2016 5:07 PM    Report this comment

First, know the bottom area of your boat. Measure the waterline-keel distance at a number of stations and integrate the area. It may be simpler to measure the keel and rudder separately. In my case this matches up to one coat per gallon.

Mix the heck out of it. Conventional paint mixers don't work as well as a joint compound mixer with a cut-down shaft. I use a cross blade mixing impeller (common in labs) which works even better when badly settled. Use a corded 1/2-inch drill from some real power and figure on 3-5 minutes per can. You will need to pour 1/3 of the product into a second can to give enough space to really mix hard. Then pour them back and forth (boxing), mixing both containers. The stuff that has settled is the stuff you need on the boat. Get it all. There should be no heavy dregs in the empty can.

Finally, apply the paint goes at a rate that gives you the stated coverage. Vary the roller pressure and how heavily you load the roller. A tiny bit of thinner (xylene) can help, particularly if some of the paint is from last time . If it went a little too far, add a second coat to the wear areas.

Easy. I doubt any 2 people get the same amount of paint off a roller, so it boils down to making the paint fit the boat, not the number of coats or the roller selected.

Personally, I like 1/4-inch nap. I get a smoother finish and coverage is about right. IF there are dings that 1/4-inch nap can't fill, that is a surface preparation issue, not a roller issue.

Posted by: Drew Frye | January 20, 2016 4:28 PM    Report this comment

Rather than junking this test could we steer the discussion towards what is is the "correct thickness" of paint on the bottom, and how to get it in practice.

Posted by: Captain Mudbank | January 20, 2016 1:15 PM    Report this comment

After using a 3/8" nap roller for years I switched to one with a 1/4" nap normally used for smooth surfaces and found it worked much better for the same reasons listed in the article, more paint on the boat. The only true test would be two hulls of the same design, painted by the same individual using the same paint but with different rollers.

Posted by: MJH | January 20, 2016 12:10 PM    Report this comment

The thing about paint is that is it like physics and the conservation of energy. It doesn't disappear it only moves around from place to place. IE, there is no free lunch.
When painting the paint either ends up on the boat, in the roller cover, or on you or the ground. Its easy to figure out where it goes. Spills are pretty obvious and rollers can be weighed before and after they are loaded with paint. The rest of it has to be stuck to the boat. I didn't know that Total Boat was made by Blue Water paints. Why the heck doesn't Jamestown say that? I've been using Blue Water paints for years. The stuff is good and cheaper than the Cadillac brands that leave you broke and homeless after painting your boat!
The goal of painting a boat bottom with ablative is to get as much paint as needed onto the bottom in the proper manner in as short of a time as possible. Trying to eek out the max sq ft per coat on an ablative paint is simply foolish. The stuff wears off! If you put on 1 mil then you only have 1 mil to wear off. So what sense is that?? Personally the best bottom paint tool I have used is a full diameter size 9 inch foam roller cover. Menards sells them among others. It lays down a nice smooth finish while carrying a decent amount of paint in the cover so the application goes fast. A hotdog type foam roller is a poor tool to do an entire bottom. Presently I'm using Blue Water bottom paint with a 45% copper load without slime protection. I might start using slime protection paint also since that seems to be my main issue right now. The 45% paint works extremely well to keep off the stupid zebra mussels in Lake Erie but the slime gets a bit thick by haulout in November now. My current routine is to buy 1 gallon of 45% paint per year, wash the bottom in the spring (it is pressure washed at haul out in the fall). Go over the entire bottom once with the foam roller and as the paint flashes, put a second coat over the leading edges of the hull, keel and rudder and that's it. That only takes about 4-6 hours total per year for the bottom. The other thing I do is to shake the paint can in a real paint shaker. I have an air powered paint shaker from Harbor Freight that works well for this. Then I stir every time I refill the paint tray. If you don't keep it mixed the copper and pigment tends to fall out pretty quickly. I have a 33 ft Pearson.

Posted by: Dave9111 | January 20, 2016 12:09 PM    Report this comment

Only valid test would be the result of a complete coat with each roller. Then you have only one soak up per coat, not a soak up every 8 oz.

Posted by: PrecisionSailor | January 20, 2016 10:56 AM    Report this comment

The only results that would be valid would be coverage with a roller after the whole boat was coated with the roller. Initial soak up is only once per coat, not every 8 oz.

Posted by: PrecisionSailor | January 20, 2016 10:55 AM    Report this comment

Kinda typical PS junk. 8 oz test out of a gallon or more typically apppied? Invalid results unless initial soak up of nap is accounted for. The issue is how much paint is applied total based on mfgr coverage for protection, not how many coats (thickness of each) which the type of roller would control?. This is a useless comparison and a waste of cyberspace. .

Posted by: KMan | January 20, 2016 9:59 AM    Report this comment

Your test correctly states the 3/8 nap roller will soak up much more paint than the hot dog roller. If the test started with both rollers clean and dry the 3/8 roller probably soaked up half the 8 oz test quantity some of which will not get on the hull. This would warp the comparison as compared to a larger test sample or starting with both rollers already loaded with paint.

Posted by: Capt tony | January 20, 2016 9:06 AM    Report this comment

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