I didn't need to use glass polish to "de-scum" the outside of my Model 3's windshield. I used a new razor blade first, then a clay bar with detailer's spray.
Here's a photo of what I scraped off with the razor blade after pre-cleaning with window cleaner and paper towels. The substance is grey in color, except for the yellow bug splatter in the center of the photo which is what I missed using the window cleaner. I scraped some areas of the windshield from the interior too, but found no such substance coating the inside.
Scraping using a new razor blade removed most of the substance, but not all. The clay bar took care of removing the remainder.
The substance is invisible under most lighting conditions. To see the scum, you need special lighting conditions which keep the interior of the car dark. I did this inside my garage with the garage door shut. For a lighting source, I used wall mounted light bulb fixtures as a lighting source.
You can only see the scum in or near the reflection of the light source. After scraping with the razor blade, the scum is even harder to see. If you apply a thin layer of window cleaner or detailer from a paper towel, the scum re-appears and resembles water spots.
Finishing off removing all the remaining "scum" with detailer spray and clay bar. The detailer spray acts as a lubricant for the clay bar, but to get the final layers of scum, you will want to use the clay bar without any lubricant, while applying detailer's spray from a paper towel in order to see where the remaining scum removal is needed.
Don't forget to do the area under the wiper blades with the car controls set to "Wiper Service Mode".