Here is what my dad found out a long time ago. We used to try EVERYTHING to get heavy spots out of windows, etc and some products worked ok but never got every spot off. My dad went to a boating store and got this rubbing compound for boats, etc (cars too) and used it. Get a sponge wet, put a tad on there. Rub in circles and don't push real hard.
Works the BEST and get's every spot off. Those other products will work fine but wont get all the real old spots off as well.