Another thing that can be used to make exterior glass smooth and free of contaminents is Whink (a laundry product found by the stain removing stuff in the grocery store). It's a very mild acid and you just wet a paper towel, very wet, and lightly rub a 1' square area in circles till all of the patterning disappears and wipe it off while it's still wet. Then just follow it up with rain-x or wax or whatever you prefer to put on their. DO NOT use it in direct sunlight, do it in the shade.