Oh heavens, I wish I knew. This is such a common problem. I know people are going to tell you try this product or that, and in my experience, none of them work all that well. The problem is the water quality in your area, just like here in my part of Florida. Water here stains shower doors, walls, etc. I have tried many different products and NONE of them work all that well. What I have found is that it usually takes 4-6 cleanings before I make a small dent in this problem, so I always tell the owner this ahead of time.