[Doug in NJ]

Are you trying to preserve the finish of the frame? Is the glass in place?
If "yes", it will probably be a pretty tedious job. You might try sharpening a small screwdriver to make a narrow chisel, and slowly chip the stuff away. If "no", you can use more aggressive methods :-)

I recently needed to get the old hardened rubber out from my side window glass channels (the metal part that goes across the bottom of the glass). I threw all of them in the barbecue, set to high, for 30 minutes. All the grease, paint, and rubber was cremated, and they just need a brief session in my bead-blaster to remove the rust and scale.

Doug