Before polishing concrete, every crack, control joint, and defect must be inspected and filled. Left untreated, they’ll stand out in the final finish or worse — cause structural failure under heavy traffic. This guide covers how to properly fill joints and cracks in a concrete slab before polishing, including which materials to use, how to apply them, and when to do it in the polishing sequence.
Start by walking the entire floor and identifying all control joints (planned cuts), shrinkage cracks, and any surface spalling. Use a torch or backlight if needed — even hairline cracks can show in polished finishes. Mark joints that need to remain functional (movement joints) and cracks that are cosmetic or structural. Don’t rely on surface appearance alone — probe with a blade or pick to check depth and width.
Any dust, debris, or moisture inside the crack or joint will prevent bonding. Use a vacuum with a crevice tool or blow out with compressed air. For deeper cracks, use a chisel or grinder to slightly widen the profile for better product penetration. The edges must be dry, dust-free, and ready to accept the filler without separation. Do not wash with water — moisture slows curing and weakens bond strength.
Use polyurea or semi-rigid epoxy joint filler for most polished concrete prep. These materials cure quickly, flex slightly with slab movement, and can be shaved flush. Avoid soft, flexible sealants — they won’t hold up under polishing machines. For wider cracks or chips, use repair mortar or sand-filled epoxy. Choose a fast-curing system if grinding will start soon after. For colour matching, some fillers can be tinted or come pre-coloured to match exposed concrete or aggregate.
Dispense the filler into the joint using a dual-cartridge gun or pour method, depending on the product. Fill slightly above the surface line — known as “overfilling” — to ensure it can be shaved perfectly flush once cured. Work in short runs to avoid waste and ensure consistency. If using a pourable filler, control the bead and avoid pooling or shrink-back at the joint edges.
Most polyurea and epoxy fillers set in under an hour, but always check the manufacturer’s specs. Once cured, use a floor razor or flush-plane shaver to remove the excess material — leaving the surface level. Never begin polishing before the filler is fully cured. If you grind too soon, heat or pressure may pull the filler loose or smear it across the surface.
After shaving, begin with your initial cut (usually 30–70 grit metal bond diamonds). If the filler is slightly harder than the surrounding slab, it may polish differently — so use overlapping passes to blend. For high-end finishes, some installers feather out these areas with hand pads or densifier to match the surrounding polish level. If needed, lightly touch up filler post-grind before final polish.
Some control joints — especially around structural columns, perimeter walls, or slab breaks — are designed for movement. These should not be filled rigidly unless you’re confident they don’t require flex. In some polished floors, installers leave movement joints open or fill with flexible urethane sealants after polishing. Make sure to check project specs before filling every line in sight.
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