Fellow redditor mmalluck shared something you might find interesting (and some might find terrible): He used acetone vapors to polish up a set of ABS keycaps.
The victim keycaps are Drop Black on White MT3 keycaps I got on the last round of BOGO.
The process is to take the keycaps and put them in a glass or metal container that has acetone in it, BUT NOT directly in the acetone as this would melt the keycaps. Instead, you put them above the level of the acetone and let the vapors from it do the work.
Keycaps ready to polish (before top pic)
Malluck used a piece of cardboard that was too wide to sit down in the bottom of a pyrex pan and then covered everything with a glass cutting dish.
I put the container outside where it was nice and warm for 30-40 minutes and this was enough to melt the surface layers of ABS.
After removing the keycaps from the container do not touch them for at least an hour (or you'll leave fingerprints) while they set.
Keycaps stewing in acetone vapors in the 90F heat
In malluck's opinion it's better to go too short than too long with the process as you can always put the keycaps in again. If you go too long though, you can permanently damage the caps (melt them).
The end result: Keycaps that are shiny – some surface texture of the MT3 caps remained though.
As commenters pointed out, this may work only for ABS caps and not PBT ones.