Funny! I lived in this very nice apartment many year ago. I tried very hard to get these gray stains out of my tub, but to no avail. One day, when I was going through the trouble with my legs (that turned out to be a magnesium deficiency) this woman told me to soak in a tub with a cup of vinegar. Well, while I was in the tub, all the gray yuck on the bottom of the tub loosened up (it was icky) and I was able to scrub it clean. It's amazing what vinegar does. I realized the cleaning power of vinegar!