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like acefalcon stated, try using diluted vinegar first to see if it will remove the calcium deposits (water spots). If that doesnt work, you can move to a light cleaner/polish like P21s Paintwork Cleansing Lotion. IF this still doesnt work, chances are, you'll need a stronger, abrasive polish to help remove the spots. http://www.glimmerglassdetailing.com/poglco.html
You can really go as strong as you'd like. 1:1 should be a good starting point though. Remember to rewash afterwards though using clean water and a quality auto shampoo.