On Wednesday, 28 October 2020 11:16:07 UTC, Dave Plowman (News) wrote:
Was grouting some new tiles, using the combined waterproof adhesive/grout,
and got called away before finishing off. The actual grouting is fine -
but a lot of residue on the tile faces, now rock hard. Tiles are ceramic
and matt finish. Any tips on the easiest way to remove the residue?

acid