About 10 posts (and 2 years ago!) I mentioned my 'epiphany' (don't worry: I had it treated): that I had been using files and rasps designed for cutting metal or wood to shave this ABS type plastic. Looked at some auto body files, hoof rasps, etc. Really happy with the 'Bahco 10-Inch FM10 File Master' - about $20 on Amazon (price fluctuates). It is like the difference between cutting with a sharp versus dull chain (!). Really helps to hold them solid in a vise.I use an axe to “dress” them, personally, but I like the idea of using a rasp/file or a sander instead.
The sander works, but you need to do 2 things for best results: 1) Sand at a slower speed, if you can, because it heats up and melts the plastic. You can sort of manage the melting by sanding into the wedge for a smooth, final finish. 2) Sand outside, if you can, to avoid exposure to the plastic 'fumes'.
Philbert