To sand concrete floors, clean, start at 30–80 grit, step to 100–200 for smoothness, and use a shroud plus HEPA vacuum for dust control.
Why Sand A Concrete Floor
Sanding a concrete slab solves three common needs: flatten high spots, open pores for coatings, and knock down surface laitance after pour or patch work. It also removes light sealers and adhesive traces that block new finishes.
Safety Gear And Dust Control Setup
Concrete dust carries respirable silica. Wear a tight-fitting respirator rated for silica work, sealed goggles, hearing protection, and gloves. Hook your grinder or sander to a shroud with a HEPA-rated vacuum. Tape doorways, run a box fan in a window to exhaust air, and keep pets and kids out until cleanup is complete.
Sanding A Concrete Floor: Tools And Grit Map
Pick the tool based on area size and the finish you want. A 7-inch angle grinder with a diamond cup handles edges and spots. A walk-behind grinder speeds large rooms. A random-orbit sander with diamond paper helps with touch-ups and corners. Pair the tool with the right grit: lower numbers cut flatness; mid grits smooth; higher grits refine for sealer or paint.
Grit Progression For Concrete
Start coarse only as needed. If the floor is mostly even, begin at 50–70. If it has ridges or trowel lines, drop to 30–40. Move one step at a time and vacuum between passes. Skipping steps leaves swirl marks and open scratches that telegraph through coatings.
| Grit Range | Purpose | Typical Tool |
|---|---|---|
| 16–30 | Heavy removal; flatten ridges and humps | 7" grinder with metal-bond cup |
| 30–40 | General leveling; remove trowel lines | 7" grinder or walk-behind |
| 50–70 | Blend and refine coarse scratches | Grinder with metal-bond cup |
| 80–100 | Smooth for paint or thin epoxy | Grinder or random-orbit with diamond |
| 150–200 | Refine for clear sealer or tight finish | Resin pad on grinder or polisher |
Step-By-Step Method
- Prep the room. Pull baseboards, mask outlets, and lift thresholds. Sweep, vacuum, and degrease stains. Mark high and low areas with a pencil and a long straightedge.
- Set up dust control. Mount a full shroud on the grinder and connect a HEPA vacuum with auto-filter-clean if you have it. Check airflow by running the tool in the air; the shroud skirt should draw in.
- Coarse pass for flatness. Fit a 30–40 grit diamond cup. Work in overlapping arcs, about one-third overlap, keeping the machine moving. Glide across seams and feather edges. Stop and check for ridges with the straightedge.
- Mid pass for scratch refinement. Swap to 60–80 grit. Repeat the same pattern. If you plan to apply a thin epoxy or sealer, this pass is where most of the scratch work is set.
- Fine pass for coating bond. Use 100–150 grit to open pores without polishing slick. If you’re aiming for a burnished look under a clear sealer, add a 200 grit pass.
- Edge and corner work. Switch to a smaller grinder or random-orbit with matching diamond discs. Blend edge bands into the main field so you don’t leave a halo.
- Dust cleanup. Vacuum slowly in two directions. Wipe surfaces with a damp microfiber pad. Check walls and sills.
- Moisture and pH check. Tape a plastic square to the slab for 24 hours to spot vapor. For a coating project, use the product’s moisture test method and a pH strip; many coatings call for pH 7–10.
When To Choose Wet Grinding
Wet grinding tames dust and keeps diamonds cool, which can speed removal on hard troweled slabs. Plan for slurry: squeegee into a wet vac with a fine filter bag rated for cement waste. Let the slab dry before testing adhesion or laying a coating.
Choosing Diamonds And Abrasives
Metal-bond diamonds cut aggressively and last on tough surfaces. Resin-bond diamonds refine scratches and bring a tighter finish. On a basic floor prep job for paint or sealer, a metal-bond cup for 30–80 passes and a resin pad for 100–200 hits the sweet spot. For small touch-ups, diamond paper discs on a random-orbit sander save time.
Reading The Surface With CSP
Many coatings specify a concrete surface profile, or CSP. Light coatings like thin epoxies often want CSP 1–3. Thicker overlays need a rougher face, up to CSP 5–9. Grinding usually lands around CSP 2. Use molded CSP chips or a chart to match your finish to the spec. See the ICRI guideline for profile ranges and selection.
Edge Cases: Glue, Paint, High Spots, And Soft Caps
Old mastic and paint can clog diamonds. Score a crosshatch with a scraper, then use a PCD segment or a coarse metal cup to break the film before switching back to standard grits. For sharp humps along a cold joint, start with short, slow passes. If the top layer is soft or dusty, apply a densifier, let it react, then sand again.
Coating Prep: From Sanded Slab To Finish
After sanding, the slab should feel even and slightly gritty. That tooth helps paint, epoxy, or clear sealer grab. Vacuum, tack rag, and do a small tape-pull test: stick painter’s tape, burnish it, and peel. If dust transfers, clean again. Follow the coating maker’s spread rate and recoat window. Avoid over-polishing before paint; a slick face can reduce bond.
Dry Time, Moisture, And Weather
Concrete can hold moisture in the core that slows coating cure. Run fans and keep windows cracked to help airflow, but avoid humid days if you can. If you cut the floor wet, wait until the color evens out and readings are within the product’s range.
Noise, Neighbors, And Housekeeping
Grinding is loud and produces fine dust. Choose daytime hours, warn neighbors in shared buildings, and seal vents lightly to keep dust out of ducts. Keep a second set of disposable booties by the door so you don’t track fines through the house.
Troubleshooting Scratch Patterns
Straight lines or “tiger stripes” point to uneven pressure or skipping grits. Circular haze often means the next grit didn’t fully remove the deeper scratches. Use a pencil to scribble a test area; grind until lines vanish uniformly. Short passes at 45 degrees to the last direction help blend.
Diagnosing Surface Hardness
If the cup skates and barely cuts, the slab is hard; drop a grit or switch to a softer bond. If it digs and sheds slurry fast, the slab is soft; raise the grit and try a densifier between passes. A simple steel nail test gives a quick read: if the nail bites easily, expect a softer surface that loads tools.
Managing Edges, Steps, And Transitions
Mask base walls with a low-tack film. For steps, use a smaller diameter cup and keep the tool square to the nosing. Where a room meets tile or wood, set a straightedge on the joint and feather the concrete so there’s no toe-stubber ridge after new floor goes down.
Silica Rules And Safe Work Practices
Dust from grinding carries respirable silica. For jobsite rules, see OSHA’s construction standard page on respirable crystalline silica. It outlines controls like tool shrouds, wet methods, and HEPA vacuums.
Common Myths That Waste Time
Myth: coarse grit always speeds the job. Reality: too coarse leaves trenches you must chase later. Myth: more water is always better. Reality: a light wet cut controls dust; a flood makes slippery slurry and hides your scratch pattern. Myth: you must reach mirror-polish for sealer. Reality: most sealers bond best to a fine, open face, not a gloss.
Pro Budgeting And Time Estimates
A single room (200–300 sq ft) with light waves often takes a half day with two to three passes. Heavy trowel ridges or patch buildup can turn into a full day. Consumables include diamond wear, vacuum bags, filter cleaning tabs, and plastic for masking. Renting a walk-behind grinder rarely costs less than a strong 7-inch kit unless the room is large.
Sanding Indoors Without A Mess
Stage gear on a plywood sheet by the entry. Lay sticky mats, run a corded HEPA vac, and empty the bin before it’s packed. Vacuum the tool body and shroud before you carry it out. Wipe the path to your exit. A clean exit keeps the rest of the home tidy.
When To Call A Pro
If you see map cracking, hollow spots, or a slab that rings when tapped, grinding may expose a repair that needs a full patch. If the job needs CSP 5 or above for thick overlays, a walk-behind grinder with more weight is safer and faster.
| Symptom | Likely Cause | Fix |
|---|---|---|
| Swirl marks after paint | Skipped grit steps | Drop back one grit, regrind, then step up |
| Dust trails from shroud | Loose skirt or poor vacuum | Tighten skirt; check hose, filter, and airflow |
| Glossy patches that repel sealer | Over-polished face | Scuff with 100–150 grit; re-clean |
| Uneven edges or halo band | Edge tool not blended | Feather 12–18 in. into the field |
| Slow cutting on hard slab | Bond too hard | Switch to softer bond; lower grit briefly |
| Loading and smear | Soft surface dusting | Apply densifier; let react; regrind |
Post-Sanding Checklist
- Floor is flat and uniform with no raised lips.
- Scratch pattern is even with no deep tracks.
- Edges blend into the field with no halo band.
- Dust is removed; tape-pull shows no residue.
- Moisture and pH are in range for the finish.
- Coating plan is set with tools and recoat time.
Care And Maintenance After Sealer Or Paint
Place felt pads under furniture, avoid harsh cleaners, and keep sand and grit at entry points with mats. Reseal on the schedule the product recommends. If a clear sealer dulls, a light 150–200 grit screen and a fresh coat restores sheen without a full grind.
Method Notes And Constraints
This guide centers on home and light shop rooms, not heavy industrial plants. Local rules on dust disposal and noise may apply. Always read the tool and coating manuals. The grit ladders here match common products; if your system lists a target CSP, match the steps to that range.