Slick61

If it's a buildup (i.e. ABOVE the surface) of hard water stains, claying & machine polishing should remove it. If it's etching (i.e. BELOW the surface of the surrounding paint) due to acid rain (or whatever), then, you're likely getting into wet sanding. Machine-polishing alone could improve the appearance somewhat, but full correction would likely require wet sanding... which, from the sound of it, you should probably leave to a professional. Depending on the depth of the etching (and the depth of your clear coat), it may or may not be correctable. Talk to a pro first.