Like I stated before, you wouldn't/couldn't see the difference before a new glass is installed.
Everything under the sun will show its age. Some things more than others. If you're not doing a whole repaint and such the used glass is fine. New paint is even more reason to replace that glass. Side glasses and rear glasses usually get by with a good polish.
When the butyl is hot its more adhesive. Since I choose to save the gaskets, I peel the gasket away from the glass inside and out with a plastic tool and apply wd-40 from a spray bottle to prevent re adhesion temporarily. Water or any chemical is not good for the glass at its edges so wash the wd-40 or whatever away a.s.a.p.