: I know PSA but what's the film?

It's pretty much what Mike says. The trade name is 3M Microfinishing Film. The backing is polyester film and the abrasive particles range from 9 to 100 microns ... so it's mostly a polishing product. The stuff I've seen and used is as smooth as a baby's butt ... way, Way, WAY less course than 2000 wet/dry sandpaper as an example ... really amazing stuff. It comes in sheets, discs and rolls and with or without the PSA.

The sharpening fanatics use PSA strips attached to a 3" thick granite reference block that's flat to .0002" and sharpen free hand or with a jig ... also seems to be the hot set-up for flattening backs. They only use a couple of grits because the abrasive particles cut super well and the film seems to be durable and lasts quite a while. The reference block runs $50 to $100 depending on the size and the PSA film is $5 per 4 X 24 sheet. It's certainly more expensive than wet/dry sandpaper and a free piece of glass ... but it's still pretty cheap and works very well indeed. If I was gonna do scarry sharp ... this is they way I'd go. Japan Woodworker has all the stuff.

Ken