Hey guys (and gals) new to the forum but not w/c. WFP's are a real way to clean windows. One needs to know the hardness of the local water and that has been covered here. The difference between R/O water and DI water is the final outcome...how clean is the water for our purposes. A typicall ro unit will reduce TDS to around 8 to 20. wherein a di unit will reduce TDS to zero.

Now, one can get excellent results using DI water...it does take experince however. Not much but enough to learn that wfp work is a bad idea on very windy days or on the sunny side of the glass. The water dose NOT spot....what happens is that the dirt or dust or whatever is ON the glass will re-adhere and THAT will spot. Or...the dust in the air will mix with the water and spot. There are ways around all of this and, if needed, I can help.