Terrazzo Lobby Damage From Acid Cleaning
A 400 m2 terrazzo lobby floor in a bank has lost gloss and shows dull etch rings, white pinholes, and dark grout lines. A previous contractor used a pH 2 acidic cleaner at 1:10 and a 200-grit screen on a 175-rpm machine, then applied a sealer that is now peeling in sheets. The terrazzo is cement-based with marble chips, 20 mm thick, and has 5 mm brass divider strips every 1.2 m. A test patch with a 400-grit diamond pad improved gloss but left visible scratches under raking light. The lobby must remain open during business hours; work is limited to 6-hour night shifts. The owner wants a wet look without a coating and has 3 nights available before a board meeting. Cleaning staff report the floor feels rougher near the etch rings than in the center.
1. Diagnose the damage and decide whether the sealer must be removed before honing. Explain how to test the coating and what methods are safe for the terrazzo and brass strips.
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The acid cleaner at pH 2 etched the cement matrix, and the 200-grit screen left scratches. The peeling sealer is a coating that must come off before honing; otherwise the diamonds will load, the coating will smear, and the new polish will be uneven. Test the coating in an inconspicuous area: apply a compatible coating stripper, cover with plastic if the label allows, and check after the dwell. Do not use an acidic stripper on cement terrazzo; it will etch the matrix further. Use a floor machine with a stripping pad or a diamond-impregnated pad, wet-vacuum the slurry, and rinse. Protect brass divider strips with tape or avoid direct heavy abrasion; acids and strong strippers can discolor brass. If the coating is a penetrating sealer rather than a film, test a small area with a high-alkaline cleaner; if no film lifts, honing may proceed.
2. Outline the restoration sequence, grit progression, pinhole repair, and owner communication for three 6-hour nights.
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Night 1: remove the sealer, rinse, and hone with 100- or 200-grit diamond depending on scratch depth, then 400. Keep the floor wet or use dust extraction. Night 2: fill pinholes with a matching terrazzo grout, cure, then hone 400 and 800. Night 3: polish 1500 and 3000, clean, and apply an impregnating sealer if specified. Test a patch first; the 400-grit test that improved gloss but left scratches shows the floor needs a finer sequence and more passes at 800 before 1500.
Three 6-hour nights may be enough for a test area, not all 400 m2; prioritize the board-meeting visible zone and schedule the rest. Tell the owner that a wet look without a coating depends on diamond polish and sealer, and it will not hide deep etches completely. Do not promise a coating-like film. Maintain dust control and barricades, and allow dry time before opening.