Stubbon Mildew
 
Stone in a dark shaded area can have constant mildew and mold problems. Cleaning the stone will only temporarily get rid of the mildew. What you need is a mildewcide to prevent it from returning. One of the best mildew preventive compounds is the mineral Zinc. Zinc has been used for years in the roofing and painting industry to inhibit mildew growth.  If you go into any paint store you can buy a paint additive for latex paint that is a mildewcide. Simply mix it into a gallon of water and apply it to the stone. Allow it to soak in and remove any excess. This should help inhibit mildew as well as other bioloigcal growths such as mold, lichens, algae,etc
 
Dr Fred

Chief Technical Director