I frequently encounter hard water stains on every single house we do. It is because no one over here get's their windows cleaned any more often than 3/6 months.
Everyone over here has sprinkling systems; always gets hardwater water stains baked on the window by the intense sun and heat.
This is my regime:
Vinegar with a bit of dish washing detergent mixed with soap scum remover. (Mineral deposit remover). There are various brands but I use the cheap stuff.
We first go around and spray all of the windows on the stained areas and let it soak. Then we go back, clean with regular solution(believe it or not, I use dish detergent w/ alcohal) or GG4. Then a scrapper.
In the extreme of the extreme, I have actually used hydrocloric acid in a very diluted strenth. Then the same: soak; scrub; scrape; clean then squeegee.
I have had success alway's with this method. I guess that is why we charge $10 per window; in and out (and the screen); tracts, stain off, paint off, etc.
I have tried MDR and it is a different approach. I personally like the acidic dissolving of the stain; scraping also to get it all; and simple cleaning again after all that.
You would be surprised at how great vinegar is. Also the simple soap scum remover you can buy at the dollar store works great.
Friday we are doing a 2 story; intriquete interior design/location; I am charging $337.00. She did not flinch at the rate. 33 windows.
Love and hugs, Debbie