Instead of using harsh chemicals to clean hard water and soap scum from glass shower doors, use non stick cooking spray. Not only is it easy to apply; once you're done it prevents scum from sticking for about a week.
How to Clean a Glass Shower Door with Non Stick Cooking Spray
This works just like magic. Spray the inside of your glass shower doors with non stick cooking spray. A fairly light overall spray works. You will see the scum and hard water residue start to disappear almost immediately.
Wait five to ten minutes, then go back over the shower doors with paper towels or a clean absorbent cloth to remove all the residue. Turn your cloth frequently until the doors are clean, streak and lint free.
This works on chrome faucets and fixtures too. Use the same method. The non stick cooking spray leaves a thin invisible residue that will repel water and soap scum through many showers. The water beads up on the glass and chrome and it will stay cleaner longer.
Avoid getting the spray directly in the tub because it will make it slightly slippery. If your doors are really bad and need a heavy application of spray, either set some old towels down first, or clean the tub afterwards with a few drops of dishwashing detergent and hot water to remove the oil.