For scratches on the back of negatives, run your finger nail down the side of your nose to extract a small amount of grease and rub this on the back of the film before you scan it. It fills the scratches with the same refractive index as the film base and they disappear during the scan.
This tip was given to me by the late, great Victor Blackman, chief photographer of the Daily Express newspaper. It works!
scratch removal for 35mm b w scans - Leica User Forum