Before you try the cerium oxide, I'd suggest you get yourself a tube of Mothers water spot remover.  I use on my cars in the Spring and mid-summer to get the last bit of gunk out of the glass (especially the front)  and get it crystal-clear again.  Cerium oxide and any other powders are what I'd call destructive, so  I'd recommend against it unless you need to remove some ripples or pits.

 

Colgate tooth powder is about the same thing as the detergent you put in your dishwasher, but a slightly finer grind.

you would get pretty impatient if your goal was to mess up glass with cerium oxide. As I said before mixing it with a polish will work great for what your doing. Removing pits and ripples would require something stronger.

Hard water spots on the glass or paint can be removed by using a wet cloth of viniger on the spot.  Simple and easy.  Once the glass is clean, Rain-X is applied.

 

Bonzi Lon

I never knew u could use viniger for water spots. Thanks bonzi

My first car sat for years. Somebody told me to use an onion to clean the glass, literally cut it in half and rub on the window. I HATE onions, but it did cut through the nasty film on the glass.

 

Read recently to use Bon Ami cleaner on glass. That worked but it wasn't as fantastic as I was hoping.