Slippery Rock Gazette

Bob Murrell 
M3 Technologies
Photos by Bob Murrell

A restoration contractor recently worked on a terrazzo exposure and light restoration project. The terrazzo had been previously covered in carpet but was recently covered in hardwood flooring. Another flooring contractor had removed the wood planking but in doing so, had scored through the wood into the terrazzo with his circular saw in quite a few places throughout the 2k square-foot project. It looked like a bunch of tic-tac-toe games had been played on the floor with the circular saw! 

The client did not want to spend exorbitant amounts of money on a full restoration but was happy with essential-only repairs (restoration hardware?). There would be large bound rugs in most of the rooms, so the floors did not need to be completely restored. However, all of the deep cuts (probably at least 1/64˝ – 1/32˝ deep) that would be exposed around the edge of the bound rug, needed to be rectified.

This was a real problem because, as you probably know, the surrounding surface would have to be ground down to the lowest valley of the cut. Terrazzo is typically poured to only ¼˝ to ½´ thick, on top of the concrete bed.

You can take a Makita hand tool with a 5˝ ELF 40 grit flexible metal-bond diamond and go to town. Of course, this would make for a big bowl of a cut which simply spreads the dip or cut in this case, out over a larger area. There is also the risk of creating chatter marks with a diamond this aggressive as most techs will try to speed up the scratch removal/blending process by running the diamond on an angle, instead of concentrating on keeping the diamond flat/parallel to the surface. It takes longer to feather out the scratch correctly, but patience pays off in the long run. The higher in the grit series the floor is taken (especially if polished), the more these refinishing imperfections show up, at the end.

Also, when you do run the floor machine to do the overall heavy grinding, the low areas are harder to hit or make good contact with. I definitely recommend using the diamonds with rubber spacers to help them follow the floor better. Let’s face it, the floor is only as flat as the original installer took the time to make it. It is common to find both original grind spots and scratches in older and newer terrazzo. Most installers typically applied a topical coating/sealer of some type which tends to hide these installation flaws. Stripping the coating and refinishing exposes them again. If you are going to finish naturally (little-to-no coatings), these need to be rectified as best possible, or they will show up like a sore thumb at the end.   

Above, Left: Two  25 lb. weights help maximize down pressure when running Flexible Metal Bond ELF3-40 grit diamonds. Above, Right: ELF3-40 grit Flexible Metal Bond diamonds for heavy stock removal.

Edges are another concern. You need to do the borders or edges (along the wall or other obstacle) before using each grit with the floor machine. If you don’t, you run the risk of leaving a picture frame around the border, whether it be lighter or darker.  So, if you are starting at 120 grit, do the borders with a 120 grit first, then do the floor machine with the 120 grit. Follow the same procedure for the next grit, and so on.

The terrazzo floor in this particular project had some difficult issues to overcome. In particular, the circular saw blade cuts imparted during the hardwood overlay removal, as mentioned above. These cuts were unfortunately quite deep in many areas and required heavy stock removal to help eliminate them completely. So ELF3 40 grit (3˝ flexible metal bonds) were used under the floor machine to accomplish this daunting task. Two 25 lb. horseshoe weights were used on the machine, bringing it up to well over 150 lbs. Three weights could have been used on the M3 Heavy Duty machine which could have sped up the process some, but operator fatigue would be much higher at this weight. Therefore, two weights were used.   

Above: Two views, and closeup: Deep circular saw cuts in original terrazo flooring from solid wood overlay removal. This was a 2,000 square foot project.

Below, Left: Damage to the terrazzo includes left over glue from wood floor removal and bad tool mar scratches from the original install. Below, Middle: Damage also included shifting eruption in the door threshold. Below, Right: Cutting in the borders with a Makita and ELF5 40 grit flexible metal bond.

The cuts didn’t go to the wall, so the borders did not need a 40 grit cut. The floors were ground with the 40s, next the borders were cut using the Makita and a 5-inch 70 grit (ELF5-70) due to original grind marks left behind, then the floors were cut with the 3-inch 70 grit under the M3 machine.

The borders were then cut with 100 grit DICP5 resins, then the floors with DICP3 100 grit, then 200 grit was used for both. The floor looked pretty good at this point and because the client did not want a super high polish or gloss, a 1500 grit PLP DIP (diamond impregnated pad) was used and the floor really started to look good. Finally, a PLP 12,000 grit DIP was used to clean everything up and the client was extremely happy with the results. 

This floor, as mentioned previously, had original grind marks in many places which were left behind by some type of hand tool or edger machine. These were probably left by some type of cup wheel. So, with all of these issues, grind marks, blade cuts, cracks and settling issues, the coarse grind with the 40 grit was totally necessary. Considering the issues on these floors, the project turned out really nice and the client was very satisfied with the results.

So, with grind marks, blade cuts, cracks and settling issues, the coarse grind with the 40 grit was totally necessary. Considering the issues on these floors, the project turned out really nice and the client was very satisfied with the results.

As always and before beginning any new project, I recommend submitting a test area to confirm the results and the procedure, prior to starting a stone or hard surface restoration/maintenance project. Also, the best way to help ensure success is by partnering with a good distributor, like BB Industries, that knows the business. They can help with technical support, product purchase decisions,  and other pertinent project information.

Bob Murrell has worked in the natural stone industry for over 40 years and is well known for his expertise in natural stone, tile, and decorative concrete restoration and maintenance. He helped develop some of the main products and processes which revolutionized the industry, and is currently the Director of Operations for M3 Technologies.

Send your comments and questions to Bob at attn: publisher@slipperyrockgazette.net .