I live in northern Illinois and we are able to use our deck for approximately 5 months starting in May. I have been using Corte-Clean every spring for 5 years and this year I'm planning on a fall application so I can use a lower concentration next spring. I have been using a 3 scoop per gallon mix. I apply the Corte-Clean with a pump up sprayer and leave it on. The first spring rain after application removes the black mildew stains and even seems to bring the appearance of the decking material back to its original condition. I'm very pleased with the end results and ease of application of the Corte-Clean composite deck cleaner.