At the end of the day if you scrub the glass and use enough water to remove the dirt then all glass
will come up perfect using purified water.
Certain types of glass will take a bit more scrubbing because as Danny has said the water beads on the glass
and rinsing wont remove all the dirt.
99% of problems will be down to what the glass is held in ( frames, seals, drain holes, vents and so on ) some of these
problems can be overcome by using different techniques but some will just cause problem after problem and are
best done using traditional methods or removed from your round.
What percentage of problem windows you get on your round will depend on luck, some might only get a few here and
there and some will get a lot, but anybody who says they have never got a problem window or that all windows can be
cleaned using wfp don't know what they are talking about.
One piece of good advice I got on here was... Know your windows.