I was reading about how citric acid is an excellent dishwashing additive for dishwashers that leave hard water stains, so I thought I would buy it to see if it would do the trick for my glass shower doors. Glass shower doors are such a nightmare to keep clean, and while daily maintenance helps a great deal, it still needs a good elbow-greasing clean every once in a while. I mixed 1 part citric acid to 6 parts hot water, and mixed it into a spray bottle. I sprayed on my shower door, and with very minimal effort, the scum seemed to slide right off the shower. In Houston, we have EXTREMELY hard water, and I have tried many other things for the shower. Oven cleaner, CLR, every purposeful shower cleaner, etc. But nothing has worked as well as citric acid and water. Next time, I'm buying the bigger container - it looks like citric acid and I will have a long, beautiful relationship!