When I built my super-insulated home in 1984, my plan was to have a walkout basement with a finished concrete slab that would absorb the sunlight from four south-facing windows and act as a thermal mass. I placed a 4-inch slab over a 3-inch layer of sand and 6 inches of XPS insulation, and used an integral red dye in the concrete. In addition, I used a wax-sealing product made by the dye manufacturer that was supposed to match the slab's color and allow the concrete surface to be buffed to a high shine.
I'm not sure what went wrong, but when we tried to buff the wax sealer, it completely lifted off the slab, exposing a soft chalky surface. Repeated scrubbing didn't solve the problem, and I ended up covering the floor with carpet and putting the kids' bedrooms down there. Over the years, laundry turned pink by the kids' clothing served as a constant reminder of this dusty yet structurally sound mess beneath the carpet.
Twenty-five years or so later, the kids were gone, and my wife and I wanted to refurbish this lower level. Thinking once again about my original idea for a finished slab, I bought a hand-held Metabo wet polisher — a right-angle grinder that feeds water onto the surface — a diamond grinding wheel, and a series of diamond honing and polishing discs, and went to work on a 2-foot-square section of the floor. A couple of hours of work yielded a hard, smooth, brick-red finish with multicolored exposed aggregate that you could see your reflection in. But the prospect of doing 1,200 square feet at 2 square feet per hour with a hard-to-control hand-held grinder led to one thought: Find a bigger machine.
Another wrinkle was that I was running a radiant heating loop (supplied by existing rooftop solar collectors) around the basement's perimeter. This entailed cutting 1 1/4-inch-deep grooves in the existing slab to accept the radiant piping, which we later grouted flush with the floor.
Grinding, honing, polishing. I found a rental store nearby that had the equipment I needed. I rented an Edco Contrx Model 2DP dual-disc polisher that came with a system of diamond grinding "dots" and honing and polishing pads, all of which were designed to work dry. I also rented an Edco TMC-7 Turbo Edge Grinder with a similar — but larger — system of pads, and an Edco Vortex VAC-200 vacuum.
The finishing process has three steps: grinding, honing, and polishing. Each uses progressively finer grits and creates progressively more sheen and reflectivity. Instead of first grinding with 30-grit diamond dots, I started with 70-grit dots, hoping to save time. But if I were to do it again, I would start with the beefier 30-grit grinding dots. I made a second pass with 120-grit grinding dots.
Honing came next. I began with 50-grit pads and worked my way up to 200-grit. The final polishing step uses pads that look like they have colored jellybeans glued to them. These include four progressively finer grits (400, 800, 1,800, and 3,500) and finally a buffing pad. After making a pass with the 400-grit, I started to see some real improvement. Also, I wasn't generating much dust anymore, so I stopped using the vacuum.
Crack filler and densifier. Older concrete usually has some hairline shrinkage cracks, so to help mask these imperfections, I used Certishine Fusion, a liquid filler made by Vexcon. I applied it immediately after finishing with the 70-grit grinding, using a Hudson Sprayer. Then I went back over the wet floor with the 70-grit grinding dots. The Fusion combined with concrete dust to form a slurry that filled cracks and small imperfections. This process was messy and the slurry sets up pretty quickly (in about an hour), so I kept an eye on the walls and cleaned up splashes right away.
I also applied a concrete densifier called Certishine Clear, also made by Vexcon. It hardens the concrete surface, making it more resistant to staining. I applied it with a mop after honing with the 200-grit, then waited for it to be fully absorbed (about 45 minutes).
Finishing up. After I made a final polishing pass with the buff pad, the floor looked great. It had a nice reflection with no fuzzy edges — think polished granite. Even the grouted lines blended in somewhat.
On the advice of several people who have experience with commercial concrete floors, I've since applied a sealer and high-gloss finish that took away the crispness of the shine. I tried to buff the finish up with a standard slow-speed floor buffer, but that didn't improve its luster. Frankly, I'm unsure if sealing and waxing is needed or not, but I was afraid that if I didn't seal this floor, something might stain it and never come out. I have too much invested now to take that chance.
All told, I have about $2 per square foot in expenses for machine rental, diamond products, and chemicals. I put 111 man-hours into the floor, which comes to about 10 square feet per man-hour. If I were doing it again, I think I could increase my efficiency 10% without much trouble.
John Starr is a custom builder in Littleton, N.H.
