Old concrete slabs often develop pits, spalls, and localized surface failures. If you try to resurface over them without repairing first, your new overlay will mirror every defect — leading to bumps, delamination, and ugly visible patches. In this guide, we'll show you exactly how to patch pits and spalls in old concrete before resurfacing, so you get a smooth, durable, professional finish.
Walk the floor and carefully mark pits, spalls, pop-outs, or localized deterioration. Small pits under 10mm deep can often be skim patched. Deeper spalls may require filling in lifts. Note if the damage is caused by freeze-thaw, moisture, mechanical wear, or poor original finishing. Understanding cause helps prevent future failure. Map out damage zones so nothing is missed during repair.
For larger spalls (especially ones with feathered, thin edges), use a chipping hammer, grinder, or saw to square off edges and slightly undercut the sides of the defect. This mechanical undercut creates a pocket that locks in patching materials. Simply filling over feathered spalls will almost always cause the patch edges to lift or crack after resurfacing. Strong mechanical edges are crucial for long-term adhesion.
After undercutting or opening the pits, vacuum all dust, dirt, and debris. For dirty or oily slabs, degrease first and allow full drying. Patching materials must bond to clean, dry concrete — contaminants like dust, oil, or laitance will destroy adhesion. It's better to spend extra time cleaning now than to risk patch failures later under the resurfacing layer.
Some repair mortars and patching compounds require priming before application, especially on highly absorbent or dry slabs. Use primers recommended by the manufacturer — usually water-based epoxies or polymer bonding agents. Apply primer lightly and allow it to become tacky (not wet) before proceeding with patching. Skipping priming when required leads to reduced bond strength and patch delamination later.
Choose a non-shrink repair mortar or resurfacing patch material compatible with your overlay system. Polymer-modified mortars or fast-set epoxy patches are ideal. Trowel the material firmly into pits and spalls, packing to eliminate air pockets. Overfill slightly to allow for later grinding or sanding flush. Apply in layers if depth exceeds 10–12mm to avoid shrinkage cracking. Always follow manufacturer's curing times before further surface prep.
Once patches have cured fully, lightly grind or sand them flush with the surrounding floor. Use medium grit (60–80) diamonds or sanding screens depending on the hardness of the patch. Feather edges carefully to avoid visible "steps" under the final overlay. Smooth transitions between repaired areas and original slab are critical — overlays and microcements are thin, and will reveal every bump if surface prep isn't perfect.
After grinding, inspect the entire surface under raking light. Minor air pockets, low spots, or missed defects should be spot patched now before priming or applying resurfacing materials. Taking the time to perfect the surface now saves huge problems later — overlays amplify imperfections rather than hiding them. Once satisfied, proceed with primer and overlay installation per your resurfacing system's instructions.
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