Quick Answer

Grind out old grout lines with a coarse diamond blade or grinding disc before polishing, then patch the joints with a matching repair mortar if they are deep, or continue with the normal grinding sequence if they are shallow. Grout is softer and different from concrete, so polishing over it leaves dirty, uneven lines in the final finish.

Why Grout Lines Must Go

Grout is a soft, porous material with a different hardness and color than the surrounding concrete. When you grind and polish the floor, grout wears differently, dusts, and leaves dark or light streaks along every joint. The only way to get a uniform polished surface is to remove the grout down to sound material and treat the joint like a patch of the same slab.

Removing the Grout

Use a diamond blade or a coarse grinding disc to cut along the joint and break the grout loose, working to a depth of 5-10mm or until you reach sound concrete. For wide or deep joints, a grout saw or a small angle grinder with a thin blade works best. Vacuum the loose material completely; grout dust contaminates the surface and stains the pores if left behind.

Patching Deep Joints

If the joint is deep, patch it with a repair mortar matched to the concrete's color before polishing. Fill the joint slightly proud and let it cure, then grind it flush with the coarse metal bond pass. Patched joints polish to the same finish as the slab, so the final floor reads as one surface. Shallow joints can simply be ground into the normal sequence.

Preventing New Grout Problems

If the floor is being re-grouted after polishing, use a grout or mortar that matches the concrete and keep the joints clean during the polish phase. Protect the joints during grinding with plastic or clean up slurry immediately, so grout paste never dries onto the polished surface.

Frequently Asked Questions

Q: Can I polish over grout lines instead of removing them?

A> You can, but the lines stay visible as dirty streaks and the grout dusts under traffic. Removal and patching gives a professional result.

Q: How deep should I remove grout?

A> Down to sound, clean concrete. A 5-10mm removal is typical; deep joints are patched rather than ground entirely out.

Q: What if the grout is full of sand and stones?

A> That is not grout; it may be a mortar bed. Remove it fully and assess the concrete below, then patch before polishing.

Conclusion

Grout lines are a prep problem with a clean solution: cut and vacuum the joints, patch deep ones, and grind everything flush in the coarse pass. Do that and the polished floor comes out as one uniform, streak-free surface.