Hard Water spots are very damaging to your cars paint, when left to dry. When Hard Water hits the paint surface of your car, that water bead is then hit by the sun which acts as a mini magnifying glass, and as that water bead drys the calcium, alkali, and minerals in that water bead ETCHES into your cars paint system. If you have paint protection such as a sealant, wax, or polish this will help to minimize damage, but its best to dry the hard water as quickly as possible. Always keep your vehicle polished for maximum protection from water, bugs, bird droppings, and pollution in the air that settles on your car. See more Detailing Tips on my Blog  @  Nu-Sheen Blog.com   and....KEEP EM CLEAN!
This is a picture of  the  STI Subaru after I completed the Detail. 16 man hrs to remove Hard Water spots. See above Video.