Not all polished concrete starts with a brand-new slab. In many renovation and commercial projects, you're faced with an existing screed, topping, or overlay — and the client wants it polished. But old overlays come with extra challenges: weak spots, inconsistent hardness, bonding issues, and variable densities. In this guide, we’ll explain how to polish over an old screed or overlay properly, without risking surface failure or poor aesthetics.
Start by inspecting the overlay or screed very carefully. Measure thickness in multiple areas (it should ideally be 50mm or more). Tap the surface lightly with a hammer to listen for hollow sounds — which indicate debonding or weak patches. Weak sections can shatter or lift during grinding. You must identify any unstable zones before polishing starts. If the topping is too thin, brittle, or poorly bonded, full polishing may not be feasible without serious risk of failure. Consider doing small core tests if necessary.
Next, evaluate how hard the surface is. Use a Mohs hardness kit or scratch tests to check if the material is firm enough to polish. Soft overlays (Mohs 2–4) will grind too quickly, dust excessively, and struggle to hold a polish. Medium hardness (Mohs 5–6) is ideal. Also assess the density — if the screed is sandy or friable, you'll need to harden it chemically later. It's crucial to determine this upfront so you can plan densifier use and pad choices accurately.
Every contaminant must be removed before grinding. Any surface dust, adhesives, paints, curing compounds, or old sealers will interfere with polishing. Scrub the slab mechanically with a neutral cleaner, or lightly grind with very soft pads to strip away residues. If oils or stains are present, consider a deep degreaser clean first. Polishing over dirty or contaminated screeds leads to patchy finishes, inconsistent sheen, and sealer failures later — preparation is non-negotiable.
After cleaning, patch any cracks, spalls, or voids using appropriate materials. Use semi-flexible polyurethanes for cracks, and strong fine-grain repair mortars for holes and divots. Surface defects will become even more visible once polishing increases reflectivity — so they must be fixed before moving forward. If cracks aren't repaired, grinding machines can worsen them or even cause flaking and delamination around weak spots. Good patch work is essential to achieving a smooth, continuous polish later.
Start the polishing process using fine or medium-fine metal bond diamond tools — typically 50 to 80 grit. You don’t want to tear through an old screed aggressively. Use lighter machine weight and gradual passes. Monitor the surface carefully: if material removal is too fast, back off immediately. The first grind's purpose is to flatten and remove surface irregularities — not deep cuts. Make sure you work consistently across the floor to avoid low spots or over-grinding thin areas.
Old overlays are often porous and weak compared to new slabs. After initial grinding, apply a concrete densifier (preferably lithium or potassium silicate). Mist it evenly over the floor and let it penetrate fully — do not allow it to puddle. The densifier chemically hardens the surface, helping it resist further wear and polish properly. Some weak overlays may require two densifier applications for best results. Always burnish lightly after curing to remove any crystallised excess.
Move into resin bond diamond pads once the slab is hardened. Begin honing at 100 grit, progressing to 200 grit carefully. This step removes all metal scratch patterns and tightens the surface for the polishing stages. Keep passes slow and controlled. Light water (wet polishing) can help control dust but make sure not to over-wet and soften the screed. At this stage, the surface should begin developing a uniform semi-matte appearance without visible gouges or swirl marks.
Continue polishing upward through higher resin grits: 400 grit for matte, 800 grit for low gloss, 1500+ grit for high gloss. Each grit stage should fully refine the scratches from the previous one before advancing. Old screeds tend to polish very quickly compared to dense natural concrete, but you still need patience. Skipping grits or rushing will leave a hazy, uneven appearance that ruins the final look under natural light.
After polishing, protect the overlay by applying a penetrating sealer or surface guard. This locks in the polish, increases stain resistance, and improves surface durability. Apply using a microfiber applicator and allow full curing before light foot traffic. For busy commercial areas, burnishing the guard product afterward improves gloss and hardens the surface even further. Without this protection step, old screeds can rapidly lose clarity and soil easily.
Before handover, inspect the entire surface under low-angle light. Look for missed scratches, dull patches, swirl marks, or streaks in the sealer. Touch up by light rebuffing if needed. Document any natural variations in colour, patchwork, or texture to avoid disputes with clients. Old overlays will always have some natural imperfections — setting expectations early ensures satisfaction and avoids callbacks.
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