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 thats 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.
For more bottom paint removal tips, check out the Boat Bottom Restoration and Painting Guide, Volume 1 of the Painting Your Boat From Bottom to Top Series from Practical Sailor.
