I've salvaged/saved original paint parts like this, usually using 1500 and 2000 grit wet/dry. Use a cross hatch method for faster and more even removal of unwanted layers. Using something to support the paper instead of your fingers helps with consistency. So does sanding in one way strokes like using a file, not scrubbing back and forth. Edges, corners and detail lines will cut through easily. Keep the paper and surface well lubricated while sanding. Using a hair dryer or heat gun will help dry the surface faster than a simple towel to gauge progress.
The better results I've achieved were with parts which had product applied over the mostly undisturbed original paint with little or no surface prep and no primer. My heavily metal flaked Norton P11A tank cleaned up well. If the painter scuffed up the original paint much, it's harder to save original paint because there may not be much (if any) left by the time you level it. go slow. You can do a test spot to see if it's worth your trouble.
Some Blasting methods can be used as well, but it's less forgiving with inconsistent thicknesses or hardness of products. Things happen faster, good or bad.
