Some wanchor has scratch my mates silver van with a key all along one side

He's filled the scratch in with paint but wants to know the best way to finish it off as it's slightly over-filled at the minute. If he t-cuts it back, it lifts the paint out of the scratch again. He thought if wet & dry but I thought it would bring the laquer off the rest of it around the scratch.

Any painters or knowledgable people know what's best?
The scratch is a key width and goes down to bare metal.

Thanks in advance