Polishing a concrete floor on an uneven slab leads to reflections that shimmer, tooling skip, and low spots that won’t polish evenly. Flattening during the grinding phase is what makes a high-end floor look and perform like it should. This guide walks through how to identify slab high and low points, choose the right tooling and machine settings, and flatten the floor before you move into finer grit stages.
Start with a 2–3 metre straightedge, laser level, or digital floor profiler. Walk the entire slab and mark any crowns (high spots) or dips. Use chalk or spray to visually outline zones that need extra grinding. This lets you target passes where material removal is needed — not just pass blindly.
You need aggressive metal bond segments to remove material efficiently. 30 grit is ideal for heavy flattening, 50 grit for lighter wave correction. Use a medium or soft bond depending on slab hardness — too hard and the segments will glide instead of cutting. Avoid resins at this stage; they conform to dips rather than removing them.
To cut high areas effectively, increase head pressure by adjusting the grinder's weight or settings (some planetary heads allow additional downforce). Walk slower — this gives the diamonds more contact time per pass. For humps and ridges, move even slower and overlap passes by 40% or more.
Run a full set of passes north-to-south, then turn 90 degrees and do east-to-west. This creates a cross-hatched pattern that evens out minor tilts and undulations. Make sure each pass overlaps the previous by at least one-third the machine’s width. Re-mark any persistent high spots before repeating passes.
Focus your efforts on grinding down the high spots. Don’t chase dips — especially if they’re deep. Grinding into low areas will only create more level inconsistencies and risk structural weakening. Feathering means blending transitions gradually rather than creating plateaus. The goal is functional flatness, not full-depth re-leveling.
As you grind, watch the dust trails behind your machine. A clean, even dust line across the full head means consistent contact. If the middle or edges are clean while others are dusty, you're tilted or gliding over a low/high zone. Adjust pressure or focus your next pass to balance it out.
Raised joints, cold pours, or slab lips may be too narrow for a full grinder head. Use a 125mm hand grinder with matching metal bond segments to knock them flat. Work in short arcs and check progress regularly with a straightedge. Blend at least 150mm beyond the edge to avoid a visible lip.
Once grinding is complete, reassess your flatness. Lay a straightedge in several directions or use a digital profiler to check tolerances. Industry benchmarks for polished floors are typically within ±3mm over 3m (FF35+). Spot-check known trouble zones and rework only where required.
Take notes on which zones needed more work and what tooling or pressure changes helped. This helps when dialing in your next pass — especially resin cuts, where dips and humps will throw off the finish. Flattening is a foundation step — logging it ensures you build on it consistently.
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