Quick Answer

To grind down high spots on a concrete floor before polishing, run a planetary concrete floor grinder fitted with aggressive metal-bond diamond trapezoid discs (16# or 30/40# grit) over the raised bands. Mark the highs with a 2 m straightedge or laser level, grind only the crowns with 2-3 slow passes, feather the edges so you do not leave a step, and re-check with the straightedge. Do not try to remove the whole slab in one pass — take the high down to the surrounding plane, not below it.

Why High Spots Appear on a Concrete Slab

High spots are common even on finished slabs, and identifying the cause helps you choose how much meat to remove.

• Formwork or screed error — the laser screed or boom pump left a 1-3 mm crown along a pour line.
• Trowel overworking — the power trowel packed cement paste into a ridge along control joints or bucket stops.
• Concrete splatter and laitance — over-sprayed splatter from a neighboring pour sits proud of the surface by 1-5 mm.
• Subgrade rebound — one edge of the slab lifted slightly after formwork strip.
• Patch repairs — a previous repair mortared proud of the slab.

Most high spots are only 0.5-3 mm proud, which is exactly what a single coarse grinding pass removes.

Tools and Tooling You Need

Leveling high spots is a surface-prep job, not a polish. Use the most aggressive metal bond you have, because you are cutting cement paste and small aggregate, not refining a scratch pattern.

• A planetary floor grinder (25-30 in) with a 3-head or 4-head plate.
• Aggressive Diamond Trapezoid Grinding Disc (JOY-TP-R3) in 16# or 30/40# grit, soft or medium bond for hard concrete.
• A Single Bar Segment Diamond Trapezoid Grinding Disc (JOY-TP-B1) for fast stock removal on the worst crowns.
• A HEPA dust vacuum, a 2 m aluminum straightedge, and a felt-tip marker.

For quick disc swaps between high-spot grinding and general prep, a Fast Change System Double Bar Segment Diamond Tooling (JOY-TPF-B2) saves time at every corner.

Step-by-Step Leveling Procedure

Follow this order so you do not over-grind the low areas.

• 1. Mark the highs. Lay the 2 m straightedge across the floor in both directions. Mark every gap over 2 mm with the marker. A rotary laser on a tripod is faster on open warehouses.
• 2. Dry-grind first. Run the 16# or 30/40# disc over only the marked bands, 2-3 slow passes. Keep the machine moving; do not dwell on one spot.
• 3. Re-check often. Stop after 3 passes and lay the straightedge down again. High spots disappear fast; it is easy to grind through the high into a dip.
• 4. Feather the edges. At the edge of the repaired band, lift the machine off gradually so the scratch pattern blends into the surrounding floor.
• 5. Continue the grit sequence. Once the floor is flat, proceed with the standard 60/80#, 120/150#, 200# metal-bond passes, then resin.

Choosing the Right Grit and Bond

Condition	Starting grit	Bond
High spots 1-3 mm, normal concrete	16# or 30/40#	Medium
Hard concrete with hard aggregate	16#	Soft
Soft, sandy concrete	30/40#	Hard
Splatter / laitance only (0.5-1 mm)	30/40# or 60/80#	Medium

If the disc glazes over and stops cutting, the bond is too hard for the concrete — switch to a softer bond. If segments wear away in 200 m², the bond is too soft.

Common Mistakes to Avoid

• Grinding the whole floor flat to the high spot — you will waste hours and overload the dust collector. Only grind the crown.
• Using a resin pad to remove high spots — resin pads load up and glaze in minutes on 2 mm proud concrete.
• Not feathering the edge — leaves a visible line in the final polish.
• Eye-balling flatness — on a reflective polished slab, judging flatness by eye is unreliable.

Safety Tips

• Run the HEPA vacuum at all times — respirable crystalline silica is regulated by OSHA and causes silicosis.
• Wear Z87.1 safety glasses, hearing protection (95-100 dB), N95/P100 respirator, and steel-toe boots.
• Inspect trapezoid shoes for cracks or missing segments before mounting; never run a damaged disc.
• Unplug the grinder when changing shoes.

Frequently Asked Questions

Q: How much concrete can I remove in one grinding pass?

A: A 30/40# metal-bond disc removes about 0.1-0.2 mm per slow pass on normal concrete. Plan on 3-5 passes to take down a 1 mm high spot.

Q: Should I wet-grind high spots or dry-grind them?

A: Dry-grind with a HEPA vacuum is faster and easier to inspect. Wet grinding is fine if dust is uncontrolled, but slurry cleanup doubles labor.

Q: Can I use a hand angle grinder to fix small high spots?

A: Yes for spots smaller than 0.5 m². Use a 5 in diamond cup wheel and feather the edges. Larger areas need a floor grinder to avoid wavy patches.

Q: How do I know when the floor is flat enough to polish?

A: With the 2 m straightedge under a side light, no gap wider than 2 mm shows across the floor. That is the tolerance most polishing crews accept.

Conclusion

Leveling high spots is a focused job: mark them, take them down with aggressive metal-bond trapezoid tooling, feather the edges, and verify with a straightedge. JOY TECH (Xiamen) supplies metal-bond trapezoid discs in 16# to 200# grit for all major floor grinders. For tooling matched to your machine and concrete, email [email protected].