No pic.
Light grit 120 for timber but test on a piece of timber first because you don't want disc marks in french doors, and don't touch the glass with any sort of sanding disc
Personally I would hand sand and mask up the glass first.
Just had to do it to a laundry door but only a small amount of glass.
Come to think of it I did some french doors a while back, used the belt sander on the flat areas and hand sanded against the glass.