If you have old grout, you may need to clean it with a wax stripper or heavy-duty cleaner plus a grout brush. You may need to use a bleaching cleanser on tough spots. Once you have the grout as clean as you can get it, rinse it well. Once it's good and dry, apply a coat of masonry sealer so that it won't absorb dirt in the future. If you have mildewed grout in tubs or showers, you can use your grout brush with a 1:5 solution of chlorine bleach and water. Anytime you're using bleach, make sure you don't have any ammonia-based products around and that the area is well-ventilated. Also, once the cleaning is done, rinse the area well to remove any traces of bleach. If you have colored grout, you should clean it with a heavy-duty cleaner and a grout brush. You'll want to be sure not to use a bleaching solution on colored grout. A masonry sealer can be applied to clean, colored grout to ward off future stains. If colored grout doesn't come clean with a heavy-duty cleaner, you may want to try a 1:5 mixture of chlorine bleach and water, but keep in mind that this could also remove the color from the grout.