Cracks filled after polishing show as bright lines forever. Filled before polishing, with the right product and enough cure time, most cracks become nearly invisible.

Sequence matters

1. Hone to the grit before the crack opens cleanly, usually 100-200.
2. Route the crack with a small blade if it is under 2 mm wide, so filler has a key.
3. Clean with vacuum, never with water that leaves residue in the crack.
4. Fill with a semi-rigid or cementitious filler matched to the floor colour.
5. Cure fully, usually 12-24 hours, before any diamond touches it.
6. Polish through the normal sequence; the filler cuts like the surrounding concrete if it is fully cured.

Filler choice

Crack	Filler	Note
Hairline, under 0.5 mm	Low-viscosity epoxy or cementitious slurry	Penetration matters more than strength
0.5-3 mm	Semi-rigid epoxy	Polishes flush, tolerates light movement
3-10 mm	Cementitious patch with aggregate	Match colour and hardness
Moving crack	Flexible sealant plus overlay option	Do not rigid-fill a live crack

Why filled cracks telegraph

• Filler harder than the concrete polishes prouder and reflects light differently.
• Filler softer than the concrete dishes out and collects dirt.
• Polishing before full cure smears filler across the surface.
• Colour matched wet, judged dry: filler darkens as it cures.

Cracks that should not be filled and polished

A crack that is widening, or one that mirrors on the opposite face of the slab, is structural. Filling it hides a symptom and the crack returns through the filler. Refer those to an engineer before quoting a cosmetic repair.

Charge for the cure time. Crack filling priced as a five-minute task is the reason fillers get polished early and the reason the lines come back.