No matter whether you use wfp for commercial or residential work, you have to understand a few things.
As Jim Willingham used to say, it's not window cleaning on a stick. In other words, you won't get perfect results right out of the box....you have to learn and practice what you learn. Many people buy a pole and system and come out with spots on the first job, and then blame the equipment. But realize that if this method didn't work, it wouldn't have stayed around the last 60 years. Also, wfp is not perfect for every scenario.
Also realize that a wfp will clean anything that soap and squeegee will clean. If you have something on the glass that requires a scraper and/or chemicals to remove, then a wfp won't clean it off...just like soap and squeegee won't.
Commercial windows have a rubber seal around the perimeter of the glass. This seal traps dirt and old soap between it and the glass. This isn't a problem for soap and squeegee, because you're not getting enough water up under the top seal to agitate and release the old dirt and soap. And, what little residue does dribble out, is easily handled by detailing the edge with a towel.
But, when a wfp is used on the glass, by comparison a lot of water is going onto the glass, and much of it gets under the seals. The result is the old soap residue and trapped dirt gets released, and drips down the window. This results in spots on the window.
The solution is simple, but takes practice to master.
Instead of doing one window completely from top to bottom and then moving onto the next one, just do the top frame, the top seal, and the top inch or so of glass. Scrub the top frame and seal side to side about 4 or 5 times accross and back. Then, thoroughly rinse that top frame and seal and move to the next window. Repeat until you've done at least 5 top frames, then go back to the first window. What you're doing is cleaning the frame, the seal, and the top inch of glass, and allowing drip-dry time for any residual crud to drip down the window and for the seal to dry. If you scrubbed aggressively enough and rinsed well enough, there should be next to no residue running out of the seal and down the window.
Do this for at least 5 windows before going back to the first. The cooler the temperature, the more dry time you should allow, so the more frames you should do first. On cooler days, I have done 30 or 40 frames before going back to the first piece of glass.
Now you're back at the first window, ready to clean the glass. Put your brush on the glass a few inches down from the top. Push the brush up until the top of it is roughly an inch below the top of the glass. DO NOT allow your brush to touch the top seal again, and DO NOT allow the water stream to splash onto the top seal. Doing so would re-wet the seal and could release any residue that was left unter the seal. This is why you put the brush on the glass a few inches down and move it up.
With your brush in place an inch below the top, move it side to side all the way accross the glass. Do this for 2 or 3 passes. Then, move the brush down an inch or 2, and brush the rest of the glass normally. You've now got a sort of "buffer zone" at the top, helping insure that you won't accidentally hit that top seal again. Scrub the glass up and down. On the bottom edge of the glass, let the brush rest on the bottom ledge and clean the bottom of the glass side to side, applying a little extra pressure. This will eliminate any brush marks along the bottom of the glass. If you've got a "curtain" of glass.......row upon row of stacked glass seperated only by frames....clean the bottom frame and the top seal of the glass that's just below the glass you're cleaning. This way you're cleaning the top frame and seals of the next row as you clean the glass of the top row. Clean and rinse those frames and seals as described above.
Now, move your brush back to the top of the glass, and rinse. If you have a monofilament brush, you can rinse with the brush on the glass. This is especially handy for work above 2 stories. If it is necessary for you to take the brush off the glass, do NOT pull the brush so far away from the glass that the water stream splashes everywhere. This splashing can splash onto the top seal or frame and result in wetting the seal and residue running down the window. If you've rinsed with the brush on the glass, on the final pass I lift up the trailing edge of my brush off the glass, taking care that the water stream does not go up into the top seal, and move the brush accross the glass and go to the next window.
It's a long explanation, but an easy technique. It just takes practice like anything else.
I found the hardest thing for me when I was learning was to have faith in the equipment and technique. It's hard to let go of the mindset you've gotten from years of squeegee work. But, you've got to have faith in your equipment and technique. And, having folks like me, your supplier, to call for tips helps put you over the top too.
I'll start another thread shortly about residential technique. Residential windows...as a rule...do not have the rubber seals around the edges like commercial does. But, they've got their own set of problems and solutions.