Dave,

Here is a "reprint" of a reply I posted recently on an auto newsgroup:

There is only one thing needed for removing water spots caused by hard water -
vinegar. It works equally well and safely on auto paint and is the only smart
way to remove water spots before waxing a vehicle. FYI - the moderately acidic
content of rain water will also remove water spots over time, but it is a
painfully long process.

The spots that you get from home water on your car are caused by the calcium
and magnesium deposits from the water. These should be easily removed
by any weak acid. Stronger acids work faster, but shouldn't be necessary
and are obviously dangerous. White vinegar is preferred, but any vinegar will
work. Do not dilute the vinegar.

Extremely fine steel wool can speed up the process but shouldn't be necessary.
A plastic scrubber such as the type used on Teflon pans or the type used to
speed up bug & tar removal on auto finishes is a better option on auto glass.

Soft Scrub is moderately safe and works on problems other than water spots.
But I wouldn't use such an abrasive if vinegar alone solves the problem. I've
used Soft Scrub on home windows and been happy with the results, but extremely
fine scratching on a car windshield can be much more noticable. I would
contact the manufacturer or test the Soft Scrub before using it on a
windshield.

(To test the Soft Scrub, try it on a scrap piece of very clean glass. Use a
power polisher with a very clean buffing bonnet. Continually re-hydrate the
Soft Scrub while doing a lot of polishing in one spot on the glass. Wash and
rinse well and observe the glass for fine scratches under a variety of angles
and a variety of lighting. If no scratches are observed then the Soft Scrub
should be safe, assuming that the auto glass is equally hard. Don't use
the power buffer on the auto glass!)

Once again, if vinegar doesn't remove the spots fairly easily, then the spots
are not hard water spots. And remember to wash and rinse glass well before
rubbing hard to remove anything from the glass. Glass is commonly scratched
by the abrasive action of the dirt which is being rubbed off. Towel dry the
glass before applying the vinegar so that you are not diluting the vinegar with
the rinse water left on the glass.

If you can't remove the spots with pure, undiluted white vinegar, then you
can test a very small area on the windshied with "Lime Away". This stronger
concentration of acid should definitely remove water spots quickly, although
I can't vouch its safety around auto finishes, etc. I would use a plastic
scrubber or superfine "OOOO" grade steel wool before resorting to a stronger
acid.

Even with vinegar, rinse well and moderately long when done. The acid from
the glass will run down between body panels when you rinse and you want
to flush out all of it.

Good luck,
Gideon