Actually my prints stick to the glass very well. And usually I am able to remove printed parts letting the glass cool down. However when I print some parts with a large base I need to use a spatula to remove the part. As a consequence the glass has some scratches on the surface.
I have used some surface materials with other printers like pet, Buildtak, pei, and others. On the UM2 I have never used anything on the glass, just glue stick but rarely.
My concern is not about parts no sticking to the glass, but the scratches on the glass.