Soon after I bought my aluminum cutter Lunacy it became apparent I needed to remove the heavy 20-year accumulation of hard antifouling paint from her hull. After I brought the boat from Florida to Maine all the old paint started flaking off in alarmingly huge chunks, presumably due to the dramatic change in water temperature.
My initial plan was to soda-blast the boat down to its barrier coat and rebuild from there. For those unwilling to either sand off old bottom paint, which is very labor intensive, or strip it off with chemicals, which is positively toxic and very messy, soda blasting is the current 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, you can do a DIY soda blast with rented equipment. Most folks, however, hire professionals, which is wise unless you really know how to use blasting gear. Though soda is 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. When I called around to check what it costs to blast a 39-foot boat like Lunacy, I got quotes ranging from $1,600 to $2,300. This certainly isn’t chump change, but if you’ve ever gone through the hell of stripping a boat’s bottom on your own it will probably seem a bargain.
Another interesting thing I learned is that soda blasting 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 eventually decided sponge blasting was the way to go. I was unfamiliar with the concept and wondered: How the heck can you "blast" anything with a sponge?
Sponge As A Media
The sponge in a sponge blast, 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 allows for enormous flexibility. Where soda ranks at a lowly and comparatively soft 2.5 on the Mohs hardness scale (originally formulated for use in the diamond industry), sponge media can range all the way 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 very delicate historical restoration work to really heavy-duty rust and scale removal.
What further enhances 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.
There are several benefits to this. 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. Joe 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 3 to 15 times. Considering 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 be easily 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, a very aggressive sponge blast can be performed in a light Tyvek jumpsuit.
Sponge can certainly be used on fiberglass boats, as it can be every bit as gentle as soda, if not more so. But, as Joe Sharpe points out, in most cases with a glass boat soda blasting is still preferable. Blasting a glass boat with soda, he explains, is normally faster. Also, soda is more green than sponge and it is easier (and cheaper) to dispose of the waste.
When Sponge is Best
The main reason Joe wanted to sponge blast my boat was that I ultimately decided we had to strip both the paint and the barrier coat off the bottom. Lunacy's original owner couldn't remember exactly what barrier coating he put on over 20 years ago, and it seemed a bad idea to build a new surface on top of an unknown quantity.
This meant the bare aluminum would need some sort of profile in order to accept a new barrier coat. And Joe'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, because it is more controllable. In my case, where we wanted a profile of somewhere between 2 to 4 mil, Joe selected a mix of 60- and 80-grit Silver Sponge media impregnated with aluminum oxide.
Before he went ahead with a full blast, Joe first blasted three sample patches with different profiles. Mike Goodwin, a tech expert from E-Paint (the copper-free antifouling paint I had selected), did me a huge favor and came with a special micrometer to measure these and give advice on exactly which profile would work best with E-Paint’s barrier system. It was precisely because we used sponge to blast the hull that this sort of exactitude was possible. For any sort of job like mine, where you really need to dial in precisely how aggressive a blast is, sponge is definitely the way to go.