Reading a Slab Before You Grind

The best polished floors start before any machine touches the slab. A 20-minute pre-assessment prevents the vast majority of failures and arguments. This lesson is your walkthrough.

1. Cracks and joints

Walk the whole area and mark every crack. Distinguish:

• Control joints — planned, straight, regular. These stay; they define the slab panels.
• Shrinkage cracks — random, often near panels. Wide or stepped cracks need repair (epoxy injection or routing + filling) before polishing, or they’ll show as dark lines in the finish.
• Structural movement cracks — wide, stepped, or growing. Stop. This is an engineer’s problem, not a grinder’s.

Photograph everything with a scale reference (a coin, a marker). You’ll need these for your quote and for handover.

2. Surface condition

• Dusting / powdering: Drag your hand across the surface. If it leaves a grey smear, the surface is weak and will need a stronger densifier — or may not be worth polishing at all.
• Honeycombing / voids: Pockets of air in the slab. Grind down and fill; if deep, they’ll always show.
• Efflorescence: White powdery salt deposits — a sign of moisture migration. Investigate the source before you commit.

3. Contamination

This is the most common silent killer of a polish. Look for:

• Cure compounds (waxy film from the pour) — must be stripped, or the densifier can’t penetrate and the shine will patch.
• Paint, coatings, adhesives — remove fully.
• Oil or grease — degrease with a suitable stripper; it will bleed back through a polish.

4. Moisture

For any coated or sealed finish, and increasingly for polish in wet environments, test moisture. A simple calcium carbide (CM) test or in-situ probe gives you the number. Above the limit, you need a moisture mitigation system — and that changes the whole job.

5. The pre-assessment sheet

Finish your walk with a one-page record:

Checklist: No machine runs until the slab is photographed, mapped and the contamination is identified. If you skip this, you’re pricing blind.

Key takeaways

• Spend 20 minutes reading the slab and you'll save hours of rework.
• Map every crack, joint and defect before the first cut — photograph them.
• Contamination (cure compounds, paint, oil) must be removed or it will seal in and ruin the finish.