Bottom blast

Soon after I purchased my aluminum Tanton 39 cutter Lunacy and brought her north late in the summer of 2006, it became evident I would have to do something about the bottom paint. On surveying the boat in Florida her bottom seemed reasonably fair with good paint adhesion. But shortly after I hauled her out in Maine that fall, to store her for the winter, the paint started flaking off in alarmingly huge chunks.

There was much speculation around Portland Yacht Services (PYS), where I keep the boat, as to what the cause might be. Some theorized it had to do with the big change in water temperature the boat had experienced, having come from Florida, where it spent many years, to Maine, where the water never gets much warmer than 50° F. Others believed it was merely the inevitable result of more than 20 years accumulation of non-ablative paint. (Checking with the previous owner, I learned he had in fact never stripped the boat’s bottom since first launching it in 1985.) Most likely, I think, it was a combination of the two.

My first full season with the boat I simply ignored the problem. In the spring, I scraped off as much loose bottom paint as I could, slapped on a fresh coat, then went sailing. The following winter, however, as part of a general refit to prepare Lunacy for ocean voyaging, I resolved to remove all the old paint and start anew.

Soda or sponge?
My initial plan was to have the boat soda-blasted down to its barrier coat and rebuild from there. For those unwilling to either sand off old bottom paint, which is painfully labor intensive, or strip it off with chemicals, which is toxically offensive and messy, soda-blasting is definitely the au courant method of choice. Unlike traditional sand blasting, which is overly aggressive and chews up the gelcoat on a fiberglass hull, soda blasting is very effective, but still gentle enough to leave gelcoat intact. The media used, which is simply bicarbonate of soda (i.e., baking soda), is non-toxic and silica-free. Because it is water soluble and non-hazardous, the media itself can be rinsed away from the material blasted off a hull, thus greatly reducing the volume of waste requiring disposal.

Theoretically it is possible to do a soda-blasting job yourself with rented equipment. Most boat owners, however, hire professionals, which is wise, unless you already have experience handling blasting equipment. Though soda is indeed a gentle media, it can still do serious damage to a gelcoat substrate if poorly handled. Besides, hiring a pro in most cases is not prohibitively expensive. Stacey Stone, who runs Chesapeake Soda Clean and blasts between 70 to 80 boats a year, says he can prep and blast a typical 40-foot hull in a single day for about $1,600.

Soda-blasting, however, is not the only game in town. Our local blasting expert in southern Maine, Joe Sharpe of JS Boatworks, came to have a look at Lunacy and quickly announced he thought sponge-blasting was the way to go. I was unfamiliar with the concept and wondered: How can you “blast” anything with a sponge?

But the sponge, it turns out, is only a vehicle. The media, in fact, consists of tiny chunks of synthetic sponge that are impregnated with various abrasive substances and cleaning agents, from aluminum oxide to steel grit to calcium carbonate to glass beads. In all there are nearly 20 different configurations, which affords a great deal of flexibility. Where soda ranks at a lowly and comparitively soft 2.5 on the Mohs hardness scale (originally formulated for use in the diamond industry), sponge media can range anywhere from 2.5 to a nice hard 8, depending on what substance is utilized. Abrasiveness can range anywhere from 16 to 500 grit. This makes sponge-blasting appropriate for anything from delicate historical restoration work to heavy-duty rust and scale removal.

Further enhancing the controllability of sponge-blasting is the nature of the vehicle itself. Where most blasting media, including soda, typically fracture on striking the target surface, thus creating lots of dust, and also rebound quite a bit, which causes collateral damage to surrounding objects and features, sponge flattens against its target surface on impact, captures a good bit of the material being blasted off the surface, keeps the impregnated media intact, and greatly reduces rebound.

This has several salutary effects. First, there is much less dust. Sponge-Jet, currently the only manufacturer of sponge-blasting gear and media, claims there is a 90 percent reduction in dust compared to most other blasting methods. Sharpe says it’s more like 80 (including compared to soda-blasting), which is still mighty impressive. Second, the media can be recycled. A big part of any sponge-blasting job involves shoveling spent media into a sorting machine that shakes the bejesus out of the little dirty sponges and separates contaminants from reusable media. Depending on the job, the media can be recycled from three to 15 times. In that sponge media is expensive, costing between $85 to $100 a bag (compared to about $23 a bag for soda, or even less for a more primitive media), this is very important.

Third, because there is much less rebound with sponge, it can be used in jobs where blasting must be performed in close proximity to fragile features. For example, Tony Anni of Sponge-Jet, who tells me sponge-blasting is currently making big inroads in the superyacht market, described a recent job in which a large yacht needed its topsides and superstructure stripped and recoated. In a situation like this, Anni explains, where windows, brightwork, and other features cannot easily be removed, sponge-blasting is ideal. Another advantage, points out Sharpe, is that less rebound is much easier on the operator. Where an aggressive blast with a media like sand or steel grit requires a user to wear leather for protection, an aggressive sponge blast can be performed in a light Tyvek jumpsuit.

But can sponge be used on fiberglass? For most boat owners this is a dispositive question. The answer is yes: sponge can be every bit as gentle as soda, if not more so. But in most cases with a glass boat, Sharpe points out, soda-blasting is preferable. Soda-blasting, he explains, is normally faster than sponge-blasting in this application; soda is also greener, and it is easier (and cheaper) to dispose of the waste.

A painful result
One big reason Sharpe wanted to sponge-blast my boat was that somewhere in there, as often happens, Plan A morphed into Plan B. Eric Thibodeau of PYS, who ultimately would be responsible for recoating Lunacy‘s bottom once it was stripped, didn’t like my idea of leaving the old barrier coat in place. Lunacy‘s original owner couldn’t remember exactly what he’d put on way back in 1985, but guessed it was some sort of coal-tar epoxy. Thibodeau, understandably, didn’t want to build a new surface on top of an unknown quantity, but preferred to go down to bare metal and start again from scratch. This made a lot of sense, so in the end I gave a thumbs-up for a “down-to-the-bone” blast job.

This meant the bare aluminum needed some sort of profile in order to accept a new barrier coat. And Sharpe’s rule of thumb when choosing a blasting media is fairly simple: if no profile is required (as when stripping a glass boat down to gelcoat) he prefers to use soda; when a profile is called for, he likes to use sponge. In my case, where we wanted to achieve a profile of somewhere between 2 to 4 millimeters, Sharpe selected a mix of 60- and 80-grit Silver Sponge media impregnated with aluminum oxide.

There was just one problem. Whatever that original barrier coat was, it was extremely tenacious and did not want to come off. Where originally we had hoped to get the job done in two days &mdash one day to prep and one full day to blast &mdash it in fact took a full week of blasting, well over 40 hours, to get all of Lunacy‘s bottom down to bare aluminum. No one was very happy about this. For Sharpe it was frustrating and tiring to have to work so slowly. For me it was very expensive. For PYS, which provided the venue for the job, it proved a bit messy.

For even though sponge-blasting generates 80 to 90 percent less dust than other methods, there is still an awful lot of dust leftover in that remaining 10 to 20 percent. Don’t think for a minute you can sponge-blast en plein air in close proximity to dwellings or other vessels. We blasted Lunacy inside a PYS storage shed and had fully tented her nether regions. But toward the end of the job, as Sharpe and his crew grew weary and got a little sloppy about securing the tent, enough dust escaped and settled on other vessels nearby that the PYS management took notice and complained.

Inevitably, I found myself pining for the road not taken. Certainly it would have been much cheaper and easier to leave the old (and obviously very effective) barrier coat in place. I consoled myself with research: coal-tar epoxy, I learned, can be very difficult to recoat in some situations. So ultimately there were no guarantees Plan A would have been any less painful.

A new coat of paint
There was another important question: what to put on Lunacy‘s bottom once it was naked? The active ingredient in most antifouling paints these days is cuprous oxide, which reacts galvanically with aluminum and so corrodes aluminum hulls it comes in contact with. In the good old days, before we cared about the environment, it was possible to use tin-based paints, which are extremely effective, but are now illegal in the U.S. Most bottom paints now sold that bill themselves as being safe on aluminum in fact contain copper thiocyanate, a less reactive form of copper that still may cause damage if it is not separated from an aluminum surface by an effective barrier coat.

My research on the subject brought me to ePaint. This is an ablative photo-reactive zinc-based antifouling paint (the active ingredient is zinc omadine) that is non-toxic and cannot harm either the environment or aluminum. I was encouraged by the fact that the U.S. Coast Guard uses ePaint on all its aluminum vessels. More important, it is clear that the future of antifouling ultimately lies in this direction. Like tin-based paint before it, poisonous copper-based antifouling is the focus of increasing regulatory scrutiny and likely will be banned eventually in both the U.S. and Europe.

The folks at ePaint were certainly very helpful. Mike Goodwin, an ePaint technical guru, made a special trip from Falmouth, Mass., where the company is based, up to Portland, Maine, just to inspect three profile samples Sharpe blasted on Lunacy‘s bottom. After examining the profiles with a magnifying glass and a special micrometer, he made a selection and recommended an application schedule: three coats of EP-Primer (an epoxy-based barrier coating), one flag coat of grey hard paint (EP-ZO-HP), following by a surface coat of white ablative paint (EP-ZO), with an extra coat at the waterline. White was not an aesthetic preference, but a prosaic one, as this color paint is reportedly most effective in combating bio-fouling on hulls. The extra coat at the waterline is recommended because this is where the paint is exposed to the most sunlight and wears away the fastest.

Having since lived with an ePaint coated boat for a full year &mdash with a summer season spent in Maine waters and a winter season spent in the Caribbean &mdash I can offer a tentative appraisal. As antifouling it is reasonably effective, but not as effective as poisonous copper-based paints. It is very effective at the waterline, but wears away very quickly; three or even four coats in this area is probably a good idea. On deeper parts of the hull it did a mediocre job combating slime and a good job of resisting hard growth. During the course of the year the hull was scrubbed three times &mdash twice in a cursory manner by me in a snorkel and mask and once very thoroughly by a diver. What little hard growth did appear was easily knocked off with a scraper.

Adhesion has been more problematic. On hauling the boat again in Portland a year later after returning from the Caribbean, I found there were a few discrete areas where patches of paint had fallen off. Apparently it was the flag coat of hard paint that had let go of the primer beneath it. These areas were relatively small (just two to three inches in diameter) and not very numerous. After the boat was out of water for more than a week there appeared much larger areas where the paint seemingly dried out and then flaked off in tiny fragments when disturbed. I consulted with Goodwin, and he advised that this is typical in areas where the paint has grown thin and is exposed to sunlight out of the water. Goodwin suggested this effect should be most pronounced on the side most exposed to the sun.

Goodwin recommended sanding the affected areas with 80-grit paper prior to recoating them. In that I have felt obliged to try to feather out the borders of these areas so as to maintain as smooth a hull surface as possible, I’ve had to do a little more prep work than I would have liked prior to repainting the boat. In the future I plan to prep and repaint the bottom very promptly after hauling the boat. Hopefully this will obviate the problem of old paint flaking off and will make life a little easier. n

Charles J. Doane is a freelance writer and editor based in Portsmouth, New Hampshire.

By Ocean Navigator