Q. After reading one of your columns I bought some "409" and, with a toothbrush, tried to clean the stains from the grout on my 50-year-old bathroom tile floor. It did not work and the grout is as black as ever. Is muriatic acid safe to use? If so, how? Julia Bristow Norfolk, Va.
A. Fifty-year-old black grout would be a challenge to 409, 509, or even 909, I suspect. Obviously, the stain is deep in the old grout and may not yield to any cleanser.
Muriatic acid, diluted 1 to 10 or so with water, may be worth a try, but it very likely will turn the black grout yellow. Experiment in a small out-of-way area.
Muriatic acid is dangerous, as it will burn the skin the clothes; also, the fumes are noxious. It is not something to play around with.
A tile setter would probably chip out the old grout and regrout the floor if he were faced with the problem in his own house. Such regrouting, however, depends on the condition of the tile, the grout, and the substrate. Therefore, a new tile floor might even be worth considering.