+1. Stick it down on a hard surface with double sided sticky tape to support it, then scrape with a slightly curved blade in an Xacto handle or similar so you can control the contact point to work on the tracks in the cable individually. Don't let the blade dig in, and mark a line across the cable and backing surface to guide you how far to scrape. It may also help to rough up the insulator surface with a fibreglass pencil (far more controllable than sandpaper) to key it enough that the scalpel blade doesn't skid over it.