Tosh's advice was pretty good, covered the basics anmirably
One other thing that I think all WFP users shoud do is to clean the brush head out regularly.
I think that contamimated bristles account for many of the very tiny spots you can get.
Not the larger stuff, and not the smears that Charlieboy got, I'm prepared to bet any money that the smears he got were like a 'bloom' across the glass, and that is caused from detergent residue bleeding out from the frames, often happens on a first time clean, which is why you need to go back over your work a 2nd time washing only the glass....so long as you have given gravity a time to do its work, go back to quickly and yo umay still get water running down off frames that may not be 100% clean and causing a run of spots.
I digress; after time your bristles will pick up grease or other non soluble soiling, when this gets onto the bristles of the brush the pure water is NOT going to flush it away, and where you brush the bristles against the glass you may well leave tiny amounts of this behind, causing very tiny spots to be left behind.
Not forgetting that with all the salt being spread over our roads I've no doubt that some of that will become airborn and get onto windows, which you will then pick up on your brush (yeah, I know, salt IS water soluble but it all adds up to a contaminated brush)
I am not saying this is an absolutely provable thing, I might be wrong, it really is just my personal opinion, but it is based on observation and trial and error.
The brush can of course become contaminated with debris you pick from the windows and frames, some of that is obvious and is very visible in the bristles.
I don't think the odd bit of grit makes a lot of difference in truth, if you can see it in the bottom of a bucket should you clean out your brush that way, then any disolvable solids around that bit of grit will have been flushed away, so should that bit of grit be left on the glass after you have washed them, when it dries out it will be just that, a bit of grit and NOT a spot.
The spotting from dirt that can be left behind is caused from much smaller particles that have disolved in the water and then dried out to cause the spotting.
But if you have been cleaning downstairs windows with the odd greassy fingerprint on, or some toddler has pressed their forehead against the patio door, or some drunken female has planted a kiss on the glass (don't you just hate it when they do that
lipstick is a pain to get off
)
Human grease is also a pain, I have a mate who is an industrial chemist, and he makes a detergent for swimming pools and showers and so on to specifically disolve human fat that gets left in showers or around swimming pools.
Water alone doesn't disolve this stuff.
So keep your brush nice and clean!
The accepted manner of cleaning your brush seems to be to keep a bucket of pure water on hand with a little GG3 or Unger liquid in it and to clean out your brush in this.
Works fine, but you need to spend a little longer on the first window you clean to make sure that enough water goes through the brush to rinse out any lingering detergent.
I'm experimenting with differing ways of cleaning out the bristles though.
At the moment I am using my hand sprayer to wet the bristles, to my mind you don't need to keep pluging it into a bucket of water to clean it, you just need to use a sulution that will disolve any greasy deposites that may have accumulated on the bristles.
when you start the next job the water should flush away the detergent you used.
Perhaps I'm being unnecessarely fussy...but I don't think so, I'm sure that not all spotting is merely down to the fact you haven't done your job properly.
Ian