If you build your harness correctly, you need to lightly sand the heatshrink tubing and inner edges of your shrink boots prior to applying resin epoxy to seal it up. Since most don't want to deal with having to find the best sandpaper for the job, we've done that for you!
3M brand long lasting 100 grit ultra-flexible sandpaper is just what you need. We offer it in strips and squares, in either packs of 5 or 25 pieces.
You will not find a better sandpaper, period!