Ok, so this is not going to cover up moldy grout, certainly not with a single coat. It is really meant for discolored grout. I also found it tended to run dry as I was drawing on the lines (maybe I draw too fast). However, tracing all the lines was easier than I thought it would be, and the end result looks really nice. So far, after about a month, it seems to be holding up. So: works well enough, certainly easier than regretting your tile!!