Applying a microcement, decorative overlay, or resurfacing system over concrete? You can’t skip proper surface grinding. Grinding prepares the slab by removing weak layers, opening pores, and roughening the surface for maximum mechanical bond strength. If you don't grind right — or at all — your overlay is likely to bubble, peel, or shear off under foot traffic. In this guide, we’ll show you exactly how to grind a concrete floor for overlays the professional way.
Walk the slab and identify previous coatings, sealers, laitance, or contamination. Perform water drop tests — water should absorb into bare concrete. If it beads, grinding must remove sealers. Check for cracks, spalls, and surface softness. Knowing the surface condition helps you select the right tooling and depth of grind required. Mark out cracks and damaged zones to avoid missing them later.
For overlay prep, you want light to medium surface removal. Start with 30–80 grit metal bond diamond tools depending on surface hardness and contamination. Hard concrete requires softer bond diamonds; soft concrete needs harder bond tools. If coatings are thick or elastomeric, use PCD (polycrystalline diamond) tooling first, then metal grinding afterward. The goal is a clean, roughened surface — not heavy surface gouging.
Grinding generates enormous amounts of respirable silica dust — highly hazardous. Always use industrial vacuums with HEPA filters connected directly to your grinder. Workers must wear proper PPE: P3 respirators, hearing protection, eye protection, gloves, and suitable work boots. For indoor grinding, seal off nearby rooms to contain dust spread. Good dust control improves worker safety and job quality.
Grind systematically across the floor. Overlap each pass by 50% to ensure full coverage without missed low spots. Move slowly — fast machine speeds create skip marks and uneven profiles. Grind in a north-south pattern, then east-west on a second pass if needed for complete profiling. Maintain steady pressure, avoiding dips or "donuts" caused by tilting the machine. Keep the grinder moving at a consistent pace at all times.
After grinding, inspect your surface profile. Overlays typically require CSP 2–3 (feels like medium sandpaper). Use CSP visual charts or perform fingertip testing — you should feel fine texturing without glassy smoothness. Perform water absorption tests: water should absorb rapidly, without beading. If the floor still feels too smooth or sealed, another grinding pass with rougher tooling may be needed.
After grinding, vacuum the surface meticulously to remove dust and debris. Dust left behind will compromise primer adhesion and overlay bond strength. Pay special attention to joints, cracks, and slab edges where grinders might have missed fine dust. Use hand grinders with edge tooling around walls, columns, or fixed obstacles to maintain a consistent surface profile across the entire slab.
Some overlays require immediate priming after grinding to prevent surface contamination (dust, oil, airborne particles) settling into the slab overnight. Always follow your overlay system's recommendations. Apply primers thinly and evenly to maximize mechanical and chemical bond strength. Do not delay priming more than 24 hours after grinding unless specified, or you may need to regrind before overlaying.
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