I accidentally came across some information online while searching for something else that said citric acid is an excellent cleaner for hard water stains. I was intrigued! Apparently one can purchase it in the grocery store in Australia, but I didn't have any luck locating any here so bit the bullet and ordered some online.
The information I had found recommended using a 10% solution for cleaning hard water stains, but I've since read that 6% will do the trick. Why use any more than is necessary? I put it to the test last night after mixing up a spray bottle with distilled water and the citric acid granules. Because it is an acid I did wear gloves for the measuring and mixing. Once diluted, I sprayed the shower door and gave it a good scrub. Voila! Finally a sparkly clean shower door.
Citric acid is used in foods and beverages as a preservative and/or flavoring to add tartness. It's environmentally benign so I feel good about using it when I need to without the worry that I'm putting something potentially toxic down the drain or into the bathroom air when I spray it.
Here's my recipe so I won't lose it:
Citric acid cleaner
60g citric acid granules
1L distilled water