Quick Answer: New concrete needs a cure period and a light laitance-removal pass before polishing, while old concrete needs heavier grinding to strip coatings and level wear. New slabs polish to a denser, more uniform finish; old slabs polish well but demand more prep and moisture control.

New Concrete: Cure Time and Laitance

Fresh concrete must cure before polishing, usually 28 days for full strength but at least 7 to 14 days for a light honed finish. The surface carries laitance, a soft, milky layer of fine paste, that dulls and blocks a polish. The first pass on new concrete is a light metal bond grind that removes laitance and opens the surface, then densifier, then the resin sequence. New slabs are also more porous, so densifier absorption is heavier and the final finish reads dense and even. round-segment grinding disc.

Old Concrete: Coatings and Wear

Old slabs bring paint, glue, oil stains, and years of wear, so the first job is stripping, not polishing. PCD tooling removes coatings, then coarse metal bond flattens the surface and removes the worn layer. Old concrete is often harder and more compacted, so it grinds slower but polishes to a high gloss once the profile is right. Check for soft spots, delaminated patches, and dampness before starting; those are the places that fail later.

Moisture and Repair Differences

Old slabs are more likely to carry moisture from below, so test before polishing and coating, just as you would for a new floor near grade. Patch cracks and spalls before grinding so the patches blend. New slabs can also suffer shrinkage cracks during curing, so allow them to develop, repair them, and densify before the fine passes. Both cases end the same way: flat surface, densifier, full resin sequence, burnishing.

Tooling and Sequence by Slab Age

For new concrete, start with a medium metal bond, roughly 120 to 220 grit, to skim laitance without over-removing. For old concrete, start with PCD or coarse 50 to 100 grit to strip and flatten, then step into the same sequence. A resin pad set finishes both the same way, and a double-row cup wheel handles the edge zones on either slab. Match the starting grit to the surface condition, not the slab age alone.

How long should new concrete cure before polishing?

At least 7 to 14 days for a light honed finish and 28 days for a full polish on structural slabs. Polishing too early pushes moisture into the surface and the polish goes dull.

Is old concrete harder to polish than new?

Old concrete is usually harder and more compacted, so coarse grinding is slower. But it also polishes to a dense gloss once the surface is open, and the prep effort is repaid in the finish.

Conclusion

Treat new slabs with cure time and a light laitance pass, and old slabs with stripping and heavier flattening, then run the same densifier and resin sequence. The slab's age sets the prep, not the finish, so start with the right grit and both polish clean.