First you clear these lines of, using thin sand paper. Then you pass them over with liquid glue, this will make them reappear in the form of an engraved line (in the best case) or as a different colored line. Using the tolls mentioned above and maybe scale plans of the subject you re-scribe panel lines. One more pass with liquid glue will clear the engraved lines. After glue completely dries, you sand with a very very thin sand paper, and... there you are!