Sodablasting 101

Practical Sailor compares sodablasting to other antifouling-paint removal methods.

One of the principle advantages of sodablasting is how effectively the old coating—and blasting media—can be contained.

Owning a boat is an exercise in perspective. A gale can make a sailboat feel like a canoe, while the prospect of bottom paint removal turns the same hull into something akin to a cruise ship. Old bottom paint always seems to have a split personality, with some portions of the antifouling coating flaking off the hull like a snake shedding its skin and the remainder stuck like glue. The poorly adhered portions of the hull coating encourage us to go ahead with our ambitious plans to strip the bottom, even though the well-adhered regions will slow the removal process to a snail’s pace. That’s why it’s important to be a realist when deciding whether or not it’s time to strip the bottom and deciding who is going to do the work.

The optimist picks up a scraper and heads toward the flaking areas of the hull. With just a few gratifying pushes or pulls, paint debris rains from the hull, and it seems like the job can be completed in hours, not days. The more pessimistic among us scrape away at the well-adhered paint, and discover that the walk in the park is more like a climb up the face of El Capitan. It’s the realist that adds up the surface area of paint that is ready to let go in a hurry and compares it with the area that remains well adhered to the hull—only then deciding whether or not to roll up his or her sleeves, open the wallet, or slap on another coat of paint and go sailing.

In the March 2009 issue, PS Technical Editor Ralph Naranjo tackled the challenge of removing bottom paint and a failing epoxy barrier coat on the bottom of his classic Ericson 41. The process was an extreme test of chemical strippers and sharp chisels, scrapers, and patience. (See “Practical Flashback,” on page 19.) But during each day of this DIY mountain climb, he dreamt about alternatives to the sand and stripper approach to paint removal and wondered whether or not a bottom-blaster could have cleaned the slate in hours rather than weeks. Since then, he’s been on the lookout for new trends in paint removal, and this update compares the Franmar Soy Strip chemical approach, our favorite in the earlier rounds of paint removal (March 2009, April 2008, and November 2006 issues), with the latest trends in sodablasting.

Background

Before picking a process for addressing an ailing hull bottom, you need to define your goals and recognize that there’s good reason to be as minimally invasive as possible. If all that’s wrong with the underbody is flaking bottom paint, and there’s no sign of blistering, gelcoat crazing or cracking, then the last thing you want to do is damage the gelcoat. In essence, your goal is to remove the antifouling paint and leave intact the gelcoat—and if one exists, the barrier coat. Doing so requires finesse, whether it involves a sanding process, dry/chemical scraping, or sodablasting.

The crew from Chesapeake Blasting Services arrived at 6:30 a.m. to begin sealing the work area with clear plastic.

The most vigorous approach to bottom woes is known as the “peel” process—major surgery that involves a mechanical cutter that’s used when deep blistering and delamination problems arise. This approach incorporates a power plane-like device on an articulating arm that cuts through paint, gelcoat, and even the top layer(s) of fiberglass-reinforced plastic (FRP) laminate. Left in its wake is raw FRP laminate with blisters open and craters exposed. After flushing, cleaning, and spot repairing damaged laminate, a full-scale relamination of the outer FRP skin takes place, followed by fairing, barrier coating, and bottom painting. It’s a major, high-cost job that requires intrusive FRP surgery. Fortunately, those who are looking to remove only the old bottom paint can take just the opposite tack and target the preservation, rather than the removal, of as much substrate as possible.

The slow and arduous task of hand-scraping does just that. Its destructive power is minimal, and the user learns a few tricks of the trade—such as rounding the edges of every scraper and letting the chemical stripper sit just long enough to soften the paint but not attack the gelcoat. Power sanding can also be a viable alternative for those with the right equipment and tool-handling dexterity. In order to speed up the process, 36- or even 24-grit abrasive is used on thick paint buildup by those wielding a powerful rotary/orbital sander. Those lacking the deft touch of a pro can easily put polyester divots in the gelcoat. It’s also a dirty job that many yards will not allow the DIY owner to engage in. Vacuum-assisted sanders, such as those made by Fein, are helpful in containing the inherent sanding dust. Also available are a wide range of chemical strippers covered in the previous PS issues listed above. For all of these reasons, many prefer to hire the boatyard staff or a subcontractor to tackle the bottom paint removal.

The diesel-powered sodablasting unit pumps up the velocity of aggregate, releasing even the most stubbornly adhered paint.

Sodablasting 101

Sandblasting has been around for decades and is part of the infernal thrum of every commercial shipyard. At the heart of the process is a high-volume, high-pressure air supply that can lift aggregate from a hopper and propel it through a hose to a nozzle that directs the flow just like water through a garden hose. Instead of a splashing spray, the resulting impact can scour rust from steel, or rip gelcoat and FRP skin right off a hull. In fact, sandblasting has become less popular as a prep method in major blister repairs because of how much surface destruction occurs during the cure. Sodablasting is a very different story, and the unique softness of the calcium carbonate powder yields an abrasive capacity that’s just enough to strip bottom paint but will leave the gelcoat intact.

Stacey Stone, the owner of Chesapeake Soda Clean, introduced us to the technology and techniques behind the process. In addition to contracting jobs in the mid-Atlantic region, he trains technicians and sells blasting equipment. One of his most valuable contributions has been his efforts to come up with a process that controls the invasive cloud of bottom paint debris and calcium residue that originally caused yacht yards to shy away from the process. These environmental and cosmetic concerns led him to team up with a boat-bottom tenting manufacturer, Bad Dust Containment Systems, which is run by Brian De Wolf and Roxanne Winslow in East Haddam, Conn. They make modular nylon, reinforced, zip-together tent segments that can be quickly set up around the hull of a boat and sealed with a gasket of tape. The rugged cocoon is then inflated into shape with a high-volume, low-pressure blower. These expandable, reusable containment systems save on one-time plastic sheeting costs and waste.

When it comes to the sandblasting process, Daniel Bernoulli, the Dutch-Swiss mathematician most famous for his studies in fluid dynamics, deserves lots of credit. Negative pressure induced by velocity changes in air flow lifts the powdered soda from the hopper, and it is accelerated by the high-pressure air flow on its way to impacting the surface. At the heart of the blasting kit is a diesel-powered air compressor that does the same job as its diminutive nephew on sale at Sears. But the result is like comparing a faucet to a fire hydrant. Blasting sand, soda, or any other aggregate is all about lots of air and a significant amount of pressure.

Bernoulli understood how the change in shape or contour over a specific surface can impact pressure. A partial vacuum can be created by forcing air through a contoured shape, and when linked to a hopper, the suction will allow media to be siphoned directly into the airflow. A media volume-control valve is used to vary the amount of soda allowed to be lifted into the air flow. There’s even an air-conditioned cooling loop that strips moisture from the compressed air and keeps water from impeding the abrasive process.

The nozzle on the end of the hose has a smaller opening diameter than the hose itself and further increases the air pressure, which also increases the tip velocity of the media expelled from the nozzle. The operator of the system sweeps the tip back and forth over the surface, operating the unit as if it were giant eraser able to peel away even well-adhered sprayed-on coatings. Each sweep of the tip causes accumulated bottom paint to fly from the surface. The resulting cloud of soda dust and paint residue will spread everywhere if not kept isolated by a significant tenting effort. Needless to say, the operator is swathed in a suit, gloves, and boots and connected to a remote air supply fed to a respirator, helmet, or full face mask.

Six hours after the crew arrived, the bottom of this Pearson 36 was free of coating (left), even in crannies near the rudder (center). The lead keel (above) was fully exposed for inspection and painting.

A tale of two Pearsons

In order to have a baseline for evaluating the sodablasting process, its cost and outcome, we compared two bottom-paint stripping approaches carried out on two similar-era Pearsons. The first Pearson began as a do-it-yourselfer endeavor that ran smack into reality—too little time and too much paint adhesion. The owner switched tacks, and the yard pros took over, using a combination strip-and-sand approach to deliver a ready-to-paint bottom. The second Pearson, a P36, sported a decade’s worth of well-adhered buildup that needed to be removed from the hull. The owner contracted with a freelance mobile sodablasting crew, Chesapeake Blasting Services.

The blasting crew arrived on the scheduled date at 6:30 a.m., hoping to be set up and ready to go before the full heat of a Chesapeake summer day took hold. In a matter of about two hours, they had repositioned jackstands over a plastic drop cloth, and taped and clamped an encapsulating plastic skirt around the perimeter from just above the waterline to mate with the drop cloth itself. By 8:30 a.m., the two sodablasters were ready to fire up the compressor and get started.

In both cases, the final outcome was a high-quality clean surface, undamaged by the paint removal process, with only a few minor repairs to make before barrier coating and bottom painting could be carried out. The big difference was that the Chesapeake Blasting Services crew had the job completed by 3:30 p.m. the same day, all signs of the debris and dust (from using 3½ 50-pound bags of media) had been contained and carefully removed from the site. The pros that used the more conventional chemical-stripper and sanding approach, spent three times as long to achieve a similar result, and the average costs for such work among the yards we caucused ranged from $1,700 to $2,800—all more than the $1,600 sodablasting charge.

Stacey Stone (left) and trainees go through details of setting up a boat hull for sodablasting.

Conclusion

Sodablasting is clearly a recommended means of bottom paint removal. It’s cost effective, often the least expensive of the options that involve a yard or outside contractor. Its lower-impact status leaves vital gelcoat undamaged but removes 99 percent of the paint. Many boatyards are moving toward relationships with independent blasting subcontractors, and time will tell whether or not the process grows into the preferred approach for bottom paint removal. In the mid-Atlantic, it’s a growth industry that leaves customers and boatyards happy with the results.

The Pearson 36 owner received an estimate of $1,400 to $1,600 for the work, and according to Chesapeake Blasting Services owner Mike Morgan, the vessel represented a middle-of-the-road challenge, as far as the difficulty of the job.

Just as important as what sodablasting can do, is what it will not do, and when it comes to removing epoxy barrier coats and linear-polyurethane top coats, these materials are too hard, and their adhesive quality makes them immune to sodablasting. In such cases, more abrasive media is needed. Morgan and Stone like what glass-bead blasting can do with these tougher coatings. There are a number of practitioners using this media in conjunction with air as well as water jet blasting, and in a future report, we will take a close look at more aggressive substrate removal options.

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