Mineral stains from various salts that have a limited ability to dissolve in water can pose real removal problems.
When Windex or full-strength vinegar won't dissolve them, the next step is to go to Barkeeper's Friend, which is a mild abrasive cleaner, somewhat between the super soft abrasive cleaner Bon-Ami (made of egg shells and soap) and the much stronger Comet Cleanser (which has such a hard abrasive it will scratch you glass windows!).
The BarKeeper's Friend has a middle of the road abrasive, soap, and a lot of oxyalic acid, which is a strong bleach and chemical solvent. Put it on the windows with a swet rag or sponge and rub gently. and let it set a while to dissolve the mineral salts that seem so persistent, then flush with lots of water to keep it off the hull and your wax job.
Then use the Rain-X (outside variety that helps water flow off by reducing the surface tension and simply lay flat and drain off the windshield.) NOTE: The interior form of Rain-X is quite different, and is designed to stop condensation from forming on the inside of the glass in the cabin.
Barkeeper's Friend can also be used to remove stains from and clean fiberglass, interior paint, and stainless steel, as well as other surfaces. Test for abrasion and scarring before you do too much area, and allow the oxyalic acid to do it's chemical bleaching and stain removing job. It's a great tool, just use reasonable caution when using it. Find it on the shelf in the supermarket next to the Comet and Bon-Ami.
Fiberglass a will require re-waxing after using it on the hull, etc. to restore the polished look and wax sealing protection.
Joe. :teeth :thup