Surface Preparation & Substrate Repair

Polishing is a revealing process. Whatever is in the slab gets exposed — so the quality of your prep sets the ceiling for the quality of the finish. This lesson covers the work that happens before the fine diamonds.

Step 1: Strip and clean

• Remove cure compounds, coatings and contamination with a stripper or a coarse diamond + vacuum.
• Degrease any oil or grease.
• Wet-vac the area until the surface is clean and free of slurry.

Step 2: Repair cracks and voids

• Hairline cracks: often can be left if stable, but for a high-spec finish, fill with a colour-matched epoxy.
• Wider cracks: route out the crack slightly, clean, and fill with a flexible epoxy or a hard repair mortar.
• Voids and honeycombing: chisel back to sound concrete, clean, and patch with a polymer-modified repair mortar. Match the aggregate size so the patch polishes in line with the surrounding slab.

Rule of thumb: a patch should sit slightly proud (1–2mm) of the surrounding surface, because it will grind down to level. If you flush it, it grinds below the slab and leaves a depression.

Step 3: Coarse grinding (establish the profile)

• Start with a 25–35 grit diamond to remove surface defects, old coatings and to establish a uniform profile.
• Work in overlapping passes, keep the slab wet, and vacuum between grits.
• The goal here is a flat, uniform, sound surface — not a shine.

Step 4: Verify before fine work

• Check flatness (a straightedge over the area; aim for minimal deflection for a quality finish).
• Confirm no contamination remains (a wet patch test — dark patches mean oil).
• Confirm moisture is acceptable for the finish you’re specifying.

Common mistakes

1. Skipping the coarse grit to save time — you then polish over unlevelled defects.
2. Patching flush and grinding the patch below the slab.
3. Polishing over fresh patch material before it’s cured — the patch grinds off.
4. Not vacuuming between grits — old slurry dulls the next diamond and causes scratches.

Checklist: Strip → clean → repair → coarse grind → vacuum → verify flatness, contamination, moisture. Only then do you move to the fine diamonds.

Key takeaways

• Prep is 60% of the job — the fine diamonds can't fix a bad substrate.
• Fill cracks and voids with a compatible repair mortar before polishing.
• Remove all contamination and get a clean, sound, dry surface before fine grinding.