Windshield glass is Laminated and not tempered. Laminated glass is a lot softer than tempered glass.(on a hardness meter tempered tinted glass is considered a 90. laminated tinted or clear is 65 in hardness or Durometer ) tempered glass is under strain all the time after it's been tempered. insides want to come OUT. imagine a balloon, poke it a little and it won't break, poke it more (deeper scratches) and it will break. But it can be polished a bit without hurting the surface tension. If you can catch the scratch with your fingernail , as the saying goes with older glass plant workers (me for one), "it ain't coming out of there" You can minimize it on tempered glass (or blur the edges somewhat) by using jewelers red rouge but it won't go away if you can feel it. And don't concentrate on one area you have to feather it just like paint. Laminated windshields react a lot better to polishing than tempered side glass.
