CEKIM is Correct, the 50/50 White vinegar treatment works wonders, it's the right Ph Level to remove the stain. Works great when cleaning your car towels as well, 2 cups in the washing machine will cut the wax and other stuff left on the towels.
or try
Duragloss Duragloss WSR (Water Spot Remover)
Removing surface water spots from paint film surface-
· Wash Car with Dawn Dish Soap (Removes WAX and Oils) do not do
this alot can damage clear coat over time...
· Use detailing clay to remove any `hard' surface granules
· To dissolve the alkaline-based, surface/etched
mineral water deposits
Equal parts water/distilled white vinegar/Isopropyl alcohol.
· Use a clean spray bottle and 100% cotton micro fibre cloth to apply the
solution to the paint surface
· Wipe off any residue from the paint surface and dry with a damp waffle
weave towel
· If any `water
spots' remain apply distilled white vinegar or Isopropyl
alcohol un-diluted to a 100% cotton micro fibre towel, using a
medium/heavy pressure on glass surface, for stubborn
spots use an
abrasive polish.
Apply a Clear coat Cleaner and Apply your Favorite Car Polish or synthetic Wax... Do Not use Organic Wax on clear coat paints (Like Carnuba) Meguires has a great line of synthetic Waxes that do not contain organic compounds.