I just wanted to post my experiences with polishing scratches out of the window glass on my Fiero. Like many, the PO of my Fiero had let the dew wipes deteriorate and the result was scratches on my windows. I bought a glass polishing kit consisting of sandpaper discs (80, 300, 1000, 3000, 7000 grit) and cerium oxide powder (mix with water in thin paste and polish with felt bobs).
The scratches polished out of the glass and looks great from the outside.
But when you are inside the car and look out, that polished area has a very noticeable wavy fun house mirror type of lens effect. I guess the windows are very sensitive to small thickness variation. This is worse than the scratches.
What the kit instructions say is that if the scratch is deep enough to catch your finger nail, you start with the sand paper and work your way down to the powder. The sandpaper is what did the damage to my windows. If the scratch is too shallow to be felt, then just the powder should be used. This works OK - I don't notice any obvious optical weirdness on sections I did this on.
So - The warning is if you embark on this, be very careful with the sanding process. The powder polishing is pretty safe.
BTW - a good number of my scratches were deep enough to catch my finger nail, which means the glass is pretty much un-salvageable.
I'm going to guess that if you need to use 80 grit sandpaper on your glass, you probably are better off getting new glass. You are correct...it doesn't take a lot of thickness variation to cause light defraction and make the glass tougher to see through clearly (you're turning it into an unrefined lens the moment you start polishing).
What surprised me is how nice it looks on the outside, and how weird it is from the inside. I didn't use the 80 grit. I did the 300 - 1000 - 7000. Can't imagine how bad it would be if I had used the 80 grit.
