I think you'd save a lot of sweat having it done professionally by Big Berts! Only costs 700php, according to my local carwash boys masakit daw ito sa kamay at braso.
Try strip it first if you want... P350 at concorde... pero kung di gumana hehe, sayang lang 350 mo. Use it with a lot of effort.. expect to spend mga 20-30mins per glass.
After using Strip-IT, make sure you wipe off all of the residues before you finish of washing your window. If you don't, you'll get a lot of hard to remove blue stains where the water flowed.
If you have a rotary buffer and know how to use it, you can DIY using fine to medium-cut rubbing compound followed by a glass polish.
In my case, because our water comes from deep well, watermarks can not be avoided.
I use a rotary buffer (setting number 2 to 2.5). I use either a meguiars medium-cut compound for heavy marks and 3M FineCut Compound for light to medium marks.
That works as well... but don't try that with old mitsu glass, it'll scratch.
Won't work with E34 glass as well, have no clue why it doesn't hehe.
As long as the waterspots aren't too baked on, it shouldn't be that hard. Just remember to use a NON-SILICONE product on the glass, otherwise, smear galore pag ulan.