Update on putting belt finishes on knives people give me to sharpen: still haven't really managed to get it work.
Slack belts: My problem is that for knives with with any small concavities, a slack belt doesn't work, and even when the grind is convex but fairly flat, I'm not able to get enough pressure in the middle of the blade using a slack belt. I think this may be because I can't tighten my belt enough without overloading the motor, but I don't know what's realistic to expect since it's my first machine.
Soft platen: I tried mounting various things on the platen to have something to push against, but the back of the belt just wore through them almost immediately. Plus, with any pressure, the grabbiness of the soft backing increased friction, and the belt wouldn't even run.
Scotchbrite: a belt that itself has give like Scotchbrite worked the best, but I only have ultrafine right now and it wouldn't take out any lower grit scratches. It's also a huge pain to get it to fit on my machine, and to have the motor actually run with it on. Lots of stretching is required. I might try buying the coarse Scotchbrite belts, but am still worried given Matus's experience.
(FYI,
@Runner_up)