[WesleyG]

If you live in an area where your tap water contains alot of minerals, if you let your car air dry without wiping down, calcium and other elements will stay on the paint and form deposits that are hard to remove, Try wiping the car down with some vinegar ( i havent done this before on a car, but I have done it for non car related stuff and it works well, so i dont know how good it is going to be for the paint), then do a foam wash and then a wax