Quick Answer

Overlap lines happen when a grinder stops, slows, or presses harder on one pass than the next, leaving a visible ridge or groove. Avoid them by keeping the grinder moving at constant speed, overlapping each pass by only 5-8 cm, and never stopping with the disc spinning on the floor. Use a planetary machine with balanced tooling such as double-round trapezoid discs and run the next grit perpendicular to the last. If lines appear, they are almost always removed at the next finer grit - the trick is catching them before resin polishing.

Why Overlap Lines Appear

A floor grinder removes about 0.1 mm per pass. If you stop for 2 seconds at the end of a pass, the spinning disc grinds a 2 mm deep groove. If you press harder at the overlap, the strip removes more concrete. These ridges show as bright or dark lines under angled light after polishing.

• Stop lines - ridges where the operator paused; fix by never stopping mid-floor.
• Pressure lines - darker strips where the operator leaned on the handle.
• Direction lines - consistent ridges from walking backwards; alternate direction each grit.
• Tooling lines - uneven wear on segments leaves repeating arcs; rotate shoes.

The Technique That Eliminates Lines

Walk the grinder at a steady pace (about 1 m per 2 seconds) in long continuous lanes. Overlap each lane by 5-8 cm - no more, no less. At the end of each lane, lift the disc or kill the spin before you turn around. Cross-hatch: run one grit north-south, the next east-west, so any ridge from one direction is cut by the next.

Tooling and Machine Setup

Unbalanced tooling is a hidden cause of lines. If one head has a sharper shoe than the others, that head cuts deeper and leaves repeating arcs. Buy shoes in matched sets and rotate them between heads every 500 sqm. For resin stages, use thin, flexible resin floor polishing pads that conform to the slab instead of stiff pucks that ride on high spots.

Fixing Lines That Already Show

• At metal stage: run the next finer grit perpendicular, covering the whole floor.
• At resin stage: drop back one grit, cross-hatch, then proceed.
• Light angled inspection: walk the floor at night with a work light on the deck to catch lines.
• Never burnish over lines - that locks them in.

For stubborn directional lines at the resin stage, a copper-bond transitional pad cuts through the pattern faster than a straight resin pad and bridges the grit jump.

Frequently Asked Questions

Q: Why do I see lines only after polishing?

Grinding scratches are invisible to the naked eye, but polished concrete reflects light like a mirror. Any 0.05 mm ridge becomes a bright line under lighting.

Q: Can a single-disc grinder avoid overlap lines?

Not easily. Single-disc machines leave circular swirl by design. Planetary machines cross-hatch naturally and are the standard for seamless floors.

Q: How much should I overlap?

5-8 cm for a 700 mm grinder. That is roughly one segment width. More overlap over-grinds the strip; less leaves an uncut band.

Q: Do I need to lift the grinder at the end of each pass?

Yes on entry-level machines. On remote-control planetary machines, slow to a stop and turn before the disc grabs. Never pivot a spinning disc in place.

Conclusion

Seamless floors come from constant speed, small overlaps, matched tooling and cross-hatch grits. Catch lines at the metal stage where they are cheap to fix. JOY TECH supplies matched trapezoid shoe sets and flexible resin pads engineered for seamless grinding; tell us your machine model for a balanced kit.