After your initial grind — usually with 30 or 50 grit — you’ll often find small pits, blowouts, pop-outs, or holes in the slab. These defects may have been filled with laitance, dust, or surface slurry before, but now they’re exposed. Repairing them properly at this stage is critical. This guide walks you through the process of patching and blending pits in ground concrete so your polished floor looks and performs like it should.
After your 30 grit cut, vacuum the entire floor with a fine-dust HEPA system. Once clean, inspect every square metre for pits, pinholes, blowouts, and edge damage. Mark any defects with chalk or tape — even shallow ones, as they’ll telegraph through later polish layers. Use a scraper to test the depth of defects that may look superficial.
Before you patch, clean out every hole or pit. Use a hand grinder, wire brush, or compressed air to remove any loose edges, laitance, or dust buildup inside the defect. The sides of the hole should be firm and mechanically rough — smooth edges prevent bonding. For very small defects, even a stiff nylon brush and acetone wipe is enough to prep the surface.
For shallow pinholes or surface pitting (under 3mm deep), use a flowable repair grout, slurry patch, or “grout coat” made from fine cement and polymer. For deeper holes (5mm+), use a sand-filled epoxy, rapid-set mortar, or polyurea patch. Choose a product that’s compatible with grinding and won’t shrink or soften under polishers. Avoid silicone or flexible fillers — they’ll smear and smear during further grinding.
Most repair mortars and fast-set grouts have a short pot life. Mix only what you can apply in 10–15 minutes. Use a trowel, plastic blade, or rubber squeegee to press the material into each defect. Overfill slightly and feather the edges to prevent low spots after shaving. Work in small zones rather than trying to patch the entire floor at once — accuracy is more important than speed.
Let the patch material cure completely before resuming grinding. Fast-set polyureas may be ready in under 30 minutes, but epoxy mortars may need several hours. Grinding too early can smear the repair, heat the edges, or pull it loose. Check the surface hardness with a metal scraper before continuing — it should feel like the surrounding slab.
Once cured, grind the patched area flush with your existing cut. Use the same 30 or 50 grit metal bond you used originally to ensure uniform scratch pattern and blending. If the patch stands proud or is visibly different, run additional passes to feather the edges. Don’t switch to resin pads or finer cuts until the patch is invisible under dust light.
On porous or mechanically weak slabs, a grout coat helps fill micro defects across the entire surface. Mix fine cementitious filler with polymer or densifier and apply with a squeegee. Push the material into small holes and drag it flush. This also enhances densifier penetration and helps the polish reflect evenly without pock marks.
Use a raking light or low beam flashlight across the floor to inspect your repairs. Well-blended patches should disappear in both colour and texture. If you see low dips or raised lines, touch them up before applying densifier or moving into resin polishing. Once the slab is sealed or dyed, it’s too late to go back.
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