Here's what you do for calcium and lime stains. It also works for general soap scum back at home:
You heat up 1 cup of vinegar (stove top or microwave)
Add 2T dish detergent
If you feel like it's very stubborn and you're going to need an abrasive, you can add 1T cornstarch.
Put it in a spray bottle and spray generously over the area.
Let sit one hour and wipe and rinse.
Test the vinegar on your painted surface, but I've never had this to damage the surface of anything I've tried.