Our large terrace is covered in mold & mildew. I have used a pressure washer to clean the surface but the rock is full of black blotches (mildew stains?) that are not coming out. I am trying to use enzyme cleaners such as OxyClean with some good results. This terrace is almost 25 years old and is constructed of stone on top of a cement foundation. It also has three large crepe myrtles dropping litter. Is that the source of black splotches? The morter around the stones is showing cracks. So I guess I have three questions, 1. Best way to remove stains? 2. How can the stones be re mortered? 3. Should I seal the stone after cleaning and repairs and if so with what product?