Today I sanded the rest of the fingers.
Since sanding by hand is so much work, I decided to test some power tools. Unfortunately ABS melts pretty quickly so I needed to find a good compromise between speed and sandpaper grid.
1. My belt/disc sander has just one speed I/0 and that was too fast and the ABS melted.
2. My sheet sander is way to large and the wobble makes it almost impossible to hold anything against it
3. I was successful with a Dremel 4000 set to 11 and 120 grid sandpaper:
After that, I quickly sanded it with a 220 grid sandpaper by hand
I then cleaned everything in water
...and fixed the pieces with some duct tape on large toothpicks