The keys to making this homemade grout cleaner work are the time you let it sit on the grout before you scrub, and the actual scrubbing. You should let the paste sit on your grout for at least half an hour before you start scrubbing to give it a chance to work. And when you do start scrubbing, be sure to use a stiff-bristled brush to really get the mold and gunk out of the cracks.