Quick Answer
To use a concrete rubbing stone, soak the concrete surface with water, then rub the stone in firm circular strokes to grind off fins, form lines and high spots. For a uniform rubbed finish, work a thin cement-sand slurry into pinholes as you rub, let it set briefly, then wipe with burlap and keep the surface damp to cure. Rubbing is easiest 1-3 days after the forms come off.
Key Takeaways
- A rubbing stone is a silicon carbide (carborundum) brick used to smooth formed concrete walls, columns, steps and edges.
- Always rub wet: water acts as a lubricant and keeps crystalline silica dust down.
- Use a coarse stone for fins and ridges, then a fine stone for the final finish.
- Green concrete (a day or a few days old) rubs much faster than fully cured concrete.
- Wear waterproof gloves and eye protection; wet cement is caustic and can burn skin.
When forms are stripped from a poured wall, step or column, the concrete is rarely perfect. You will see seam lines, sharp fins, bug holes and slight bulges. A rubbing stone fixes those flaws by hand and gives the surface an even, dense texture that looks finished and takes paint or sealer well.
What Is a Concrete Rubbing Stone?
A concrete rubbing stone, also called a rubbing brick or carborundum brick, is a rectangular block of bonded silicon carbide abrasive. Many have a molded handle or a groove for grip. They come in grit grades, usually a coarse stone for heavy removal and a fine stone for finishing, and some are sold as two-sided combination bricks.
Rubbing stones are designed for vertical and formed surfaces. They are not meant for finishing fresh flatwork like a slab or sidewalk, which is shaped with floats and trowels while the concrete is still plastic.
| Tool | Best use | When to use |
|---|---|---|
| Coarse rubbing stone | Fins, form seams, high spots | First pass, 1-3 days after stripping |
| Fine rubbing stone | Uniform rubbed finish, edges | Second pass, after slurry |
| Rubbing stone with handle extension | Large walls, high areas | Big vertical areas |
| Angle grinder with cup wheel | Heavy grinding on cured concrete | Large defects; needs dust control |
| Magnesium or wood float | Fresh flatwork | While concrete is still plastic |
When Should You Use a Rubbing Stone on Concrete?
The ideal window is soon after the forms are removed, often within 1-3 days, while the concrete is hard enough not to be damaged but has not reached full strength. Concrete keeps gaining strength for weeks, so the longer you wait, the more effort each pass takes.
You can still rub fully cured concrete. Expect slower progress, use plenty of water, and consider a grinder for heavy ridges before finishing by hand.
What Do You Need?
- Coarse and fine rubbing stones (or a two-sided brick)
- Bucket of clean water and a stiff brush or sponge
- Garden sprayer or hose with a mist nozzle
- Portland cement and fine sand for slurry (if filling pinholes)
- Burlap or a coarse rag for wiping
- Waterproof nitrile or rubber gloves, safety glasses, knee pads
- Plastic sheeting or wet burlap for curing
How Do You Use a Concrete Rubbing Stone Step by Step?
Step 1: Inspect and Clean the Surface
Brush off dust, form release oil and loose particles. Mark the areas that need work: fins, seam ridges, bulges and bug holes. Knock off large fins first with a scraper or by tapping gently so the stone is not doing all the heavy work.
Step 2: Soak the Concrete
Wet the area thoroughly with a sprayer or sponge so the surface is saturated but not running with puddles. Wet concrete grinds more evenly, keeps the stone from clogging, and prevents airborne silica dust.
Step 3: Rub With the Coarse Stone
Hold the stone flat against the surface and rub in overlapping circles with firm, steady pressure. Work in sections of about 2 by 2 feet. Keep the stone flat so you do not gouge dips, and re-wet the surface whenever it starts to look dry.
Step 4: Fill Pinholes With Slurry
For a classic rubbed finish, mix a thin grout of roughly 1 part portland cement to 1 to 1.5 parts fine sand with water to a creamy consistency. Many finishers add some white cement to match the color of the existing concrete. Brush or rub the slurry into bug holes, then keep rubbing with the stone so the paste and ground fines fill the voids.
Step 5: Finish With the Fine Stone
Switch to the fine stone and rub the same section again in circles until the texture is uniform and the slurry is worked into the surface. The goal is an even, dense skin with no ridges or visible form lines.
Step 6: Wipe With Burlap
Once the slurry begins to stiffen, usually after a short wait, wipe the area with dry burlap to remove excess paste and leave a consistent, slightly textured look. Do not let thick layers of slurry dry on top; they can flake off later.
Step 7: Rinse and Clean the Stone
Rinse the stone in water and scrub the face with a stiff brush so cement does not harden in the pores. A clogged stone stops cutting and polishes instead of grinding.
Step 8: Cure the Surface
Keep the rubbed area damp for several days by misting, covering with wet burlap, or taping plastic sheeting over it. Proper moist curing helps the slurry bond and reduces surface dusting and hairline cracking.
Is It Safe to Rub Concrete Dry?
Dry rubbing or grinding concrete releases respirable crystalline silica, which OSHA regulates because long-term exposure can cause silicosis and other lung disease. Keep the surface wet, work outdoors or ventilate, and wear a properly fitted N95 or better respirator if any dust is visible.
Safety: Wet cement and slurry are highly alkaline and can cause chemical burns. Wear waterproof gloves and long sleeves, keep slurry out of your eyes, and rinse skin promptly with clean water if it gets splashed.
Common Mistakes When Using a Rubbing Stone
- Rubbing on the edge of the stone. This digs grooves. Keep the full face flat on the concrete.
- Letting the surface dry. Dry rubbing clogs the stone, creates dust, and leaves swirl marks.
- Leaving thick slurry. Slurry should fill holes, not coat the wall. Thick films crack and peel.
- Mismatched color. Test a small hidden area first; gray cement slurry can look darker than aged concrete.
- Skipping curing. Rubbed areas that dry out in sun or wind can dust and craze.
What Are the Alternatives to a Rubbing Stone?
For large walls or very hard concrete, a variable-speed angle grinder with a diamond cup wheel and a dust shroud connected to a HEPA vacuum removes fins much faster. Many contractors grind first, then rub by hand to blend the finish. For filling bigger voids or spalls, use a polymer-modified patching mortar rather than slurry.
A rubbing stone only addresses the surface. If you see structural cracks, especially in foundation walls, read our guide on how to repair foundation cracks in concrete walls. Moisture showing at cracks can also point to drainage problems; see how to unclog an underground gutter drain pipe.
When Should You Call a Professional?
Hire a concrete contractor when walls have large honeycombed areas with exposed aggregate or rebar, when a finish must meet a project specification, or when rubbing large architectural surfaces where consistent color matters. Honeycombing deep enough to expose reinforcing steel can affect durability and should be assessed, not just cosmetically rubbed.
Tip: If you plan to paint or seal the concrete, let the rubbed surface cure fully and follow the coating maker’s minimum cure time, often around 28 days for new concrete.
Frequently Asked Questions
What grit rubbing stone should I use on concrete?
Start with a coarse stone to knock down fins, seams and high spots, then switch to a fine stone for the final rubbed finish. Many suppliers sell two-sided bricks with a coarse and a fine face, which covers most DIY wall, step and column work.
Can you use a rubbing stone on old concrete?
Yes, but fully cured concrete is much harder, so progress is slower and the stone wears faster. Keep the surface very wet, take smaller sections, and consider an angle grinder with a diamond cup wheel and dust shroud for large ridges before finishing by hand.
Do you need slurry when rubbing concrete?
Slurry is optional. Use it when you want to fill bug holes and pinholes for a uniform rubbed finish. If you only need to remove a few fins or soften sharp edges, water alone is enough. Wipe off excess slurry so no thick film remains.
How do you clean a concrete rubbing stone?
Rinse it in a bucket of water and scrub the face with a stiff brush right after use, before cement paste hardens in the pores. A clogged stone glazes over and stops cutting. Let it air-dry before storing.
Can a rubbing stone smooth a concrete floor?
It can smooth small rough patches or edges on a slab, but it is too slow for whole floors. For flatwork, rough areas are usually handled with a floor grinder, and fresh slabs are finished with floats and trowels while the concrete is still workable.
