Mildew on the exterior of ethanol storage tanks has been a persistent problem. Most coating manufacturers have added a mildewcide in their finish coats to arrest this problem. This approach, in my experience, has not proven very effective. PPG/Ameron manufacturers a polysiloxane, PSX700, which so far has proven far and away the best material for this application. It possesses a hard glossy finish which with its inorganic backbone minimizes this problem. It is not perfect but is much better than anything else.