I like to keep my car clean. It gets a wash at least every other weekend and a wax maybe twice a year. But washing it on a hot day is a real challenge. This weekend the sun was drying the water before I could leather it off and it's left a mass of little ring marks all over the roof and bonnet. My car is black and these marks look horrible. I can only assume it's lime scale. I live in a hard water area. Can you suggest any remedy?