It's partly uneven end-grain take up, and partly areas where more of that stuff was applied. It doesn't look like its on the surface, so don't think a heat gun will work, but its worth a try.

If it doesn't, then just attack it with much coarser sandpaper or even a file/rasp. You should only need to take 1mm off at most to get to bare wood, and you can do that without ruining the general outline/shape of the body. Then go up through the sandpaper grades, making sure that each finer grade has removed the scratches left by the previous grade before moving up to the next grade.

I'd start at 60 and see how you go.