Last spring I got it in my head to make a concrete bowl with broken bottle glass aggregate. I had a bunch of blue and green glass bottles on-hand, and broke these up by submerging them in a big galvanized washtub and bashing them with a fence-post driver. I had a book with a broken-glass concrete recipe, and I mixed up a small batch and pressed it between two stainless-steel mixing bowls. I set the stacked bowls in a corner of the porch, covered them with a soaking-wet towel, covered that with a plastic garbage bag to hold in the moisture, and weighted everything down with a pair of cobblestones. Here’s what the cast form looked like when I knocked it out of the mold a week later…
At this point, the instructions in the book I was working from advised the reader to “sand or grind the surface to expose the glass.”
I had no idea what I was in for.
I tried every kind of sandpaper in the toolbox, to nil effect: scrubbing a quarter-sized area for half an hour wouldn’t even begin to expose the glass aggregate beneath. I bought some flap-wheel abrasive disks at the big orange store and mounted them on my heirloom Sears Craftsman auto-body grinder, and didn’t fare much better. I tried sanding wet and sanding dry. I even experimented with acid-etching. The more aggressive carbide and diamond polishing media available from the local hardware outlets was all intended for use on floors. It was rigid and flat and wouldn’t work on a curved surface without grinding facets all over it.
I found this set of eight “soft” polishing pads on Amazon for $50, and took a chance. The business side of each 4″ diameter pad consists of a polymer honeycomb that looks sort of like the bottom of a sneaker. The elastomer, whatever it may be, has industrial diamond grit embedded inside. In use, the matrix slowly wears away, exposing fresh grit. The back of each pad is covered with “loop” Velcro and marked with silver numbers indicating the pad’s grit size. The Velcro is also color-coded in case the numerals wear off, which hasn’t happened to mine yet. But it’s a nice detail, anyway.
The set also includes a medium-hard rubber pad holderwith black “hook” Velcro on one side and a 5/8-11 threaded brass insert. That’s a standard grinder arbor thread and it fit my old Model 315 perfectly.
I turned the bowl upside down and fit it over an old bar stool so I could work on it standing up. My first experience was not good. My grinder only has two settings. I flipped the switch to “HI,” plugged it in, turned it on, and started polishing. The pad quickly overheated, despite my diligent efforts to keep the bowl wet while I was working, and the cement binding the Velcro to the pad holder melted and failed.
Frustrated, I complained to the seller, and was pleasantly surprised by their response, which was to A) send me a free replacement holder, B) tell me how to fix the one I already had using rubber cement, and C) explain why it failed in the first place. The new one showed up in the mail three days later, but I still actually haven’t used it because the rubber cement fix worked great and held up fine once I slowed the grinder speed down. Which I did by switching to “LO” and using a universal motor speed controller from Harbor Freight.
With my equipment problems resolved, I set to with the grinder. At the lower speed, I was able to get away without using water. I spent about 10 hours working at the largest (50) grit size, just grinding away the so-called “cream” to expose the glass, and then worked down to smaller and smaller grits, spending about two hours each on 100, 200, 400, and 800 grits. There was a really striking improvement in the step from 400 to 800, and for a day or two I fully intended to polish the whole bowl all the way out to 6000 grit. But then I lost patience and just applied a polymer sealant (specifically Arrow-Magnolia International’s Glo-crete) inside and out. This provided a nice, shiny, “wet-look” gloss.
It was still a heckuva lot more work than I counted on, but I dunno how I would’ve done it at all without this set of pads. And it looks like even the 50 grit size still has quite a bit of life left in it, as you can see from the detail photo above, which was taken after the bowl was complete.
No part of the set bore any kind of manufacturer marketing, so I don’t know anything about where they come from besides the product link on Amazon and the seller website at GranitePolishingPads.com. They are marketed as “for granite counter tops,” and honestly I have no idea how well they perform in that application. But for curved surfaces, at the proper grinding speed, they have my unequivocal endorsement.