The tile looks clean and the grout does not. You mop, the floor dries, and the lines between the tiles are still grey, because the mop rides over the grout and never gets down into it. Grout is porous. Whatever has been walked into it over the years is sitting below the surface, and a surface tool cannot reach it.

The label calls this a professional strength acidic formula, and that is the whole idea. Soap and a mop clean the top. An acid works on the stain itself: my reading of the label is that it goes after what is bound into the pores and lets go of it, so the stain lifts instead of being smeared around by a brush. That is my reading, and it stays a reading. What the listing states plainly is that it restores stained grout lines without the need for heavy scrubbing, and that it is made for both white and colored grout, which matters because a bleach approach can strip color.

The procedure is the part people skip. Apply it to the stain, let it sit and work for about three minutes, then wipe it away. The wait is where the acid does the work; wiping the second it lands is how you end up back on your knees with a brush. Ventilate the room and keep it off metal fixtures and anything the label warns against, because it is an acid and it behaves like one.

Thirty two ounces, and it is a one job bottle: grout and tile stains. It is not a daily floor cleaner and it is not for a surface acid will etch, so read the label for what it is rated on before the first pass. On a floor that is otherwise fine and only the lines have gone grey, this is the step the mop cannot do.

Priced at $8.53, rated 4.2 out of 5 from 10,480 ratings, and marked Amazon's Choice, all read off the listing on 30 Aug 2026.