My husband uses Clorox Bleach Gel (Cleaner + bleach). He wets the underside of the awning and then sprays the dark spots with the bleach/gel and then he uses a soft brush to rub off the dark marks and mildew and the rest of the awning and then rinses it really well with a spray hose and lets it dry. Our trailer is 9 years old and the awning is still white and in excellent condition. He tries not to retract the awning if it is wet or if he has to, he will put it out as soon as possible so it dries out. We have also stored our trailer outside in all kinds of weather, including cold, snow, freezing temps and the hot sun of Arizona. It still looks great.