There are many products and techniques to handle adhesion, and many recommendations from many sources. Experiment and see what works best for you.
I too followed the Prusa instructions with Windex for PETG. The results were variable, though it did help enough to avoid damage to the PEI but I didn't like the lack of consistency - some things were over-adhered and difficult to remove while others would not stick properly during printing, and it needed cleaning and re-coating frequently. I decided to try the often recommended unscented Aquanet (after testing the clumpy and uneven coating of glue stick) and it worked much better for me than any of the others, and it works for many filament types so I don't have to have a bunch of different solutions for the filament types I use.
Prusa cannot really recommend Aquanet because it is pretty much a USA only product and other hairsprays often have ingredients that prevent success. Windex and Glue stick are more universally available. Glue stick and Aquanet have basically the same active ingredient which has some adhesive properties that are unusually well suited to 3D printing. Windex is basically a weak contaminant, if you've ever tried it on a camera lens it leaves a bunch of junk behind. It doesn't provide adhesion which old tired PEI needs. It works better on new PEI, but the trick is getting the right amount of anti-adhesion from it. Glue stick and Aquanet provide a sacrificial adhesive layer which is more consistent. It is hard to get glue stick perfectly thin and smooth, but many people prefer it. Aquanet mist can be very evenly applied, and it doesn't take much. So a can lasts a very long time. And it is very inexpensive.
In the process of testing I learned to never touch the printbed with fingers (flexing the sheet or using a polished scraper to remove prints and bits) and the Aquanet coating would last for many print cycles. Finger oils contaminate the surface and cause variable adhesion problems. Since I don't touch the printbed I get a dozen or more print cycles between re-misting with Aquanet. It shows visible dust over time but the prints don't show any effects from the dust so I ignore it until I do a full cleaning cycle (rinse with water which dissolves the Aquanet and carries away the dust etc, and follow with isopropyl wipe down, dry and then re-mist) which isn't very often.
I'm still using the original smooth PEI sheet from the initial MK3 pre-order over four years ago. It has produced many thousands of prints, I've long lost count. I used to have to periodically use acetone, or brass wool, or very fine sandpaper as the adhesion was getting weak on this old smooth PEI and some types of prints were not sticking well. Aquanet solved these issues better than any of the recommended Windex or other surface preparation techniques like acetone and sanding that I tried. And it is much easier to do.
I still haven't emptied a full Aquanet spray can since only a light misting is needed, and not very often. I hardly use any isopropyl alcohol anymore, I don't need to clean between each print. Just get any material off the bed by flexing and clean up with the scraper, (never fingers) and print again. I use a stiff paint scraper, but I polished the edges and corners so it doesn't scratch the build plate PEI surface. I also have a plastic spatula for sweeping any stray bits of plastic off the bed easily and never contaminate the printing side of the sheet with finger oil. I do touch the tabs, edges and the underside when removing it for flexing, but never touch the printing area of the top surface (just like never touching a good camera lens). This technique saves me a lot of time and chemicals. I used to go through a lot of isopropyl alcohol and paper or blue shop towels, now I rarely use any. It's a different approach to handling the printing surface.
I don't mist the sheet with Aquanet directly while on the printer, I move the steel sheet away from the printer for misting, no need to get the Aquanet on the printer mechanics and frame.
However you choose to do it - happy printing!
