Yellowish stains (?) in my shower grout!
I have a natural stone tile shower with sanded grout, grayish in color. We have a new home and have been here about 6 months now. I've been having problems with grout discoloration from day 1 (that's another story), but for the past few months (maybe longer) some portions of the grout have a yellow/white looking substance that I cannot get off no matter how hard I try. I bought a grout brush from MB Stone Care as well as lots of their products--the everyday cleaner (MB5), the mildew cleaner (MP9?(, and the soap scum cleaner (MB6?). I have tried methods both recommended and NOT recommended for natural stone (out of desperation, plus it was just on the grout mainly)--vinegar, rubbing alcohol, a special enzymatic grout cleaner from Home Depot, Soft Scrub, Mr. Clean, even a diluted bleach (which I know is a no-no). Nothing even makes the stuff budge. I'm attaching pictures..has anyone ever seen anything like this? Thanks for any help you can provide.