How hard could it be to polish a section of concrete that measures a mere 6 by 12 feet? Only 72 square feet. At least that’s what I told myself as I started to polish the floor in my home’s bathroom. I made quite a few mistakes along the way. In writing this article, I talked to Jennifer Faller with QuestMark, a division of CentiMark Corp., to learn what I should have done differently.
All of the floors on the ground level of my house are integrally colored concrete. This level has bedrooms and a bathroom. Ten years ago we had Shawn Weaver with Concrete Floor Systems in Englewood, Colo., polish the floors in the bedrooms since there was a lot of efflorescence beneath the cure and seal that was applied during construction, but we skipped the bathroom since the floor looked acceptable and the cost was high. Weaver told us that the very tight space meant polishing by hand. So that was mistake No. 1: not getting the bathroom floor polished with the rest of the floor.
After 20 years living in the house, my wife decided it was time to remodel the bathroom. I suggested that while doing that we should polish the floor. We loved the other polished floors, and again, how hard could it be? I discussed it with Peter Wagner with Curecrete Distribution and he encouraged me to go ahead with the project and he even provided me with some chemicals and diamond pads.
Once our remodeler had torn out most of the bathroom fixtures, I was under the gun to get started. Armed with a Bosch variable speed grinder and a set of diamond polishing pads, I first went to Home Depot to get a sanding plate on which to mount the diamond pads. I had to improvise a bit here but soon had the Velcro attached. But as soon as I started the grinder and touched the floor, the diamond pads flew off, crashing into the walls. That was mistake No. 2.
I went online and got a standard backer plate from Polished Concrete Solutions. This was better; at least the pads didn’t go ballistic. As instructed, I sloshed on a bit of KickStart and dug in with 50-grit pads. Cutting through the sealer was the hardest part of the job, that and handling the tremendous amount of slurry generated—nasty stuff! About every 10 minutes, I would have to stop and dig concrete sludge out of the grooves in the diamond pads. When I had done about a 1-square-foot section, I would add some water and squeegee the slurry into a dust pan. I ended up with a pretty good bag full of slurry.
After about eight hours, the first step was completed. All of the sealer was off and it had a rough finish, but my back and knees were suffering. Mistake No. 3, according to Jennifer Faller, was to not strip the sealer first, although later I remembered that Concrete Floor Systems had started with cup wheels to get off the sealer, so that was another option that might have made things go more quickly.
When the concrete floor was placed 20 years ago, the finishers cut contraction joints into the wet concrete with jointing tools. For some unknown reason, they placed one of the joints within about 1 inch of the toilet drain resulting in the joint going under the toilet. The joint never activated.
After the initial grind of the floor, I decided to fill the joint with a patching material, then polish it. First, I prepped the joint with a grinder to get rid of the sealer. I used Mapei Quickpatch and added some Quikrete color to try to match the floor. I experimented with the color and finally got what I thought was a good match. I mixed enough material to fill the entire joint and then placed it such that it was higher than the adjacent floor. I ground the joint flush and it looked pretty good. In the end, the patching material didn’t really polish and the color was quite a bit lighter than the adjacent floor, which darkened considerably when I applied the densifier. Let’s call that mistake No. 4.
Grind and Densify
Moving to the 200-grit pads, I started polishing and trying to get out all of the scratches from the initial grind. I used water to keep down the dust and this step went much more quickly than the initial grind. The biggest mystery for me in this step was when it was complete. Had I polished enough? It was just a guess.
Faller later described the process to me this way: When you do the initial grind, you create tiny grooves in the concrete. During the next step you are cutting off the tops of the little pyramids, making the floor shinier. You know when you're done when the pads stop taking off material, creating dust. What I should have done was grind with the 200-grit pads then cleaned the floor and grind again with the 200-grit until it stopped creating dust. Then it would have been time to move to the next higher grit. Mistake No. 5!
I moved to the 400-grit pads, but since I was just guessing on the timing, some spots ended up with visible rounded scratch patterns that did not come out. Also, since the floor was nowhere close to flat, some high places were ground much deeper, exposing aggregate, and some remained a cream finish.
After I thought I was done with the 400-grit, I applied Retroplate as a densifier. The floor really sucked it up so I added more and let it soak in for about 30 minutes. I’d read that if there was too much it could leave white residue on the concrete so I was careful to wash it off. That step seemed to go fine and the floor started looking pretty good.
One difficult spot was blending the bathroom polish into the hallway that had been polished previously. I never did get them to blend very well—it depends on the light. So that must be mistake No. 6.
Moving to the 800-grit dry polish made the floor very shiny, but also created a plastic-looking haze on the surface, almost like wax. I tried removing that with diamond-impregnated maintenance pads but to no avail. Mistake No. 7. Faller calls this resin transfer, meaning that it is the resin holding the diamonds on the pads that is transferring to the floor. She recommended removing the resin with mineral spirits or xylene and also to clean the blackened burns off of the 800-grit pads. I tried the mineral spirits on the diamond pads and then on the floor and that worked well.
The final step was to apply RetroPel as a stain repellant. That did not seem to change the color of the concrete at all but it made a huge difference in how quickly water drops were absorbed by the concrete. I applied it with a cotton cloth and it went on like water and absorbed quickly.
In the end, despite my many mistakes, the floor looks pretty good. If I had to do it over again, I probably would still do it despite how much work it was and despite the less than perfect results. Mistake No. 1 was the worst, though, and if anyone asks I will tell them to leave this stuff to the pros.