Water Spots:
Alkaline watermarks (water spots) are calcium and magnesium salts that deposit on the surface after the water has evaporated, the minute crystals bond to the surface and are not re-dissolvable in water. These fall into one of two categories a) surface or b) below surface (etched) water spots.
a) Removing water spots from glass surfaces-mineral deposits can be caused by water from a light summer shower, or a lawn sprinkler system that that dries on the glass surface leaving a calcium / sodium deposit, these can be removed by using detailer's clay to remove any hardened surface deposits, and then using a solvent type cleaner (Klasse All-In-One) for stubborn spots polish them using method b
·Use detailing clay to remove any `hard' surface granules
·To dissolve the alkaline-based, surface/etched mineral water deposits try one or more of the following; a 5:1 solution of distilled water/distilled white vinegar, or distilled water/Isopropyl Alcohol (adjust ratio as required) or equal parts distilled water/distilled white vinegar/Isopropyl alcohol.
·Use a clean spray bottle and 100% cotton Microfiber cloth to apply the solution to the glass surface
·Wipe off any residue from glass and dry with a damp waffle weave towel
·If any `water spots' remain apply distilled white vinegar or Isopropyl alcohol un-diluted to 100% cotton Microfiber towel, using a medium/heavy pressure on glass surface.
·If this does not remove the `water spots' use Autoglym Car Glass PolishTM and #0000 or #000 steel wool, use straight-line motions only (circular motions cause swirl marks)
·For stubborn spots polish them using method b
b) Below surface (etched) water spots-
Can usually be removed using Autoglym Car Glass PolishTM and a random orbital buffer (speed # 4) to level the surface. (See also Windscreen Protective Barrier)
Do not use abrasive cleaner; glass polish or any grade synthetic steel wool on after market-tinted glass or you will probably scratch the surface.
Also you might try:
Zaino Z-12 Glass Polish