This stage usually happens when hard water has been sprayed onto a window once or twice with ground water from a sprinkler or garden hose. After the water has evaporated, visible spots are left behind.
At this stage the mineral deposit has started to build up to the point where it has a somewhat transparent, but white pasty look. At this point, it may be difficult (but not impossible) to remove with normal scrubbing
This is where the hard water deposits have build up to the point where they have etched into the pores of the glass. The scale build up has not only damaged the glass, but has damaged the frame as well. You will see this a lot of times on shower glass and doors along with lower outside windows that have been hit over and over by sprinkler systems. You will know the damage has been done when the window scraper or cleaners will not remove the scale build up, no matter how hard you try.