I keep seeing a product recommended for just this situation as well as siding and such.Wet and Forget.Never tried it. https://www.google.com/search?q=wet+and+forget&rlz=1C1CHBF_enUS760US760&oq=wet+and+for&aqs=chrome.0.0j69i57j0l4.9805j0j8&sourceid=chrome&ie=UTF-8