Based on reading others posts over the years, I'm assuming my hard water at my house will create water spots on my car if it was protected with a coating, like it does with waxes or sealants. like I mentioned above, I'd join the club if I knew a coating would keep hard water from spotting on my paint.
Hard water can be tough on waxes and sealants. In the sun it can bake on and be difficult to remove. I have hard water spotting on some glass surfaces that I can't remove.