Depending on how bad, or deep the scratch is, it can be very lightly sanded, and then polished, or possibly just polished.
A 'mild' form of polish is toothpaste, which acts just like a polishing or grinding compound. However, used sparingly, metal polish, or 'T-Cut' (car body colour restorer), often works better on more pronounced scratches or ridges. This is then followed by polishing with a soft cloth.
The series of pics here are of the canopy of my 1/32nd scale Beaufighter, the first pic showing an un-wanted frame after removal with a scalpel blade and sanding.
The second pic shows the 'T-Cut' being applied, using a cotton bud (Q-tip).
Third picture is after polishing.