Bon Ami is abrasive. Feldspar is rock. Don't use it. I got into an extended argument on the net a year or so ago with someone about this and so I contacted Bon Ami. They said DON'T USE IT on your windshield. Still, lots of people recommend it. So now it appears that they are printing on the label: DON'T USE IT. Still, that isn't enough for some people.
As to cleaning the "won't come off at all scum" from your windshield, try pure ammonia. Wear gloves, don't get it on anything but the windshield. Apply to a rag liberally then clean the windshield. Keep water handy to flush in case you get it on anything else. When you've wiped the windshield several times over, wash it all off with lots and lots of water. When you get it right, the water will sheet off instead of bead.
Don't wax your windshield. It causes haze/streaking when you use your wipers.