Clear Shell will keep your mildew from coming back, for a year and a half. Clear Shell will work inside or outside. It was first made for auto cloth tops, then awnings and upholstery. I would have given a lot to know about this product when I had a boat because I was always cleaning mildew off the cushions. It’s great for outside furniture, awnings, outside umbrellas, grout in the bath, infact I can’t think of anything that Clear Shell won’t work on.