It’s been a number of years since I cleaned glass professionally so take what I say with a grain of salt. Back when I started out I was one of those guys we joke about that got a bucket and a squeegee and just started cleaning glass. There was no internet; when I got a cell phone I kept my number private and used a pager for advertising because cell phones cost so much - and the phones were like a brick.
Point being there was no information out there that was readily accessible to someone just starting out. I never worked for a company that could teach me how to clean glass - when I discovered the window cleaning magazine it helped immensely. The good 'ole days…
Most of what I learned came from trial and Error. Learned a lot from mistakes. Ultimately after trying to hire employees and having problems finding anyone that’d do the kind of work I do consistently and show up when scheduled, I decided to just be solo and specialize in custom residential (in hindsight I should have bit the bullet and hired a crew - you know what they say about hindsight…).
Anyway, so now when I read about how folks are cleaning custom glass I have to wonder how many are scratching the exteriors of high end windows because they simply don’t know that particulates can get embedded in their scrubbers and act like sandpaper. If you’re working on homes with stucco or if concrete is on the glass the chances of doing so are greatly increased from my experience. Even dirt from landscaping can become embedded in a scrubber and then act like sandpaper.
As such, I would generally clean all exterior residential if they were an initial clean in two phases (unless by some chance they were actually being maintained properly). The first pass is to get all the heavy particulates - the best way possible without buffing the glass at all. Brushes can help keep particulates from scratching the glass when there is heavy buildup imho, and water to rinse it off of course. I then clean up all the heavy debris that could contaminate my scrubbers. If accumulation is excessive or there is a chance that large particulates could be trapped on the edge of the frame I use a piece of wool to clean up the edges, always making sure to turn the wool so it isn’t causing damage. Then throw it away in case you picked something up from the edges - bottom edges most likely to do so. Steel wool is cheap. Keep the wool you use to scrub the glass as clean as possible. Know that when your using ‘elbow grease’ to clean a window you’re not actually putting scratches in glass - aggressively so.
Only after I clean all the particulates off do I do any heavy scrubbing with a scrubber - always keep your scrubbers clean. If concrete or stucco is on glass best to rinse it off somehow to make sure you don’t pick up something on your scrubber or t-bar even. Stucco, concrete & small rocks can get get lodged in a sleeve and scratch the heck out glass. Won’t see it until you fan it out. Not what you want to see as you finish what used to be a nice window. Failing to remove heavy particulates before squeezing a window can even cause scratches if the particulates get lodged in the squeegee. No fun getting done with a window and seeing scratches that you may have caused.
Maybe I’m just a perfectionist, but after years of doing glass it just seems like making sure to remove heavy particulates before finishing a window is the safest way to clean exterior custom residential. Not only is it safer imho, but getting your windows to come out close to perfect is a lot easier if your solution isn’t dirty from heavy debris. A trace of clean solution on glass in sun isn’t going to show nearly as much as if your water is dirty.
Takes longer to do it right, but beats having to wonder if you’re going to get blamed for damaged glass. Lot of scratches are caused by careless cleaning. Better to be able to explain to a customer why you’re a little higher than the next guy because you take care to not scratch their glass than make excuses why their glass is scratched because you cleaned them wrong imho.