I have a sandstone patio here at my 80-year-old Cleveland, Ohio, home. The patio was re-done last year, but using the old stone that has been here forever. Several stones have moss on them and are very slick. A few have some black spots that look like mildew. I've heard everything from blast-'em-with-a-pressure-washer to get-a-scrub-brush-and-some-bleach for cleaning recommendations. What is the best thing to use, and how often does the patio need to be cleaned?