Card scrapers, sharpen 'em bro!

Depends what I'm using the scraper for. For cutting back old varnish, smooth single cut file and off to work. For finish prep, joint with a 1200 DMT stone held on its edge and the scraper elevated on a scrap of wood.

Dead square edges are good for bragging rights, but not really necessary. An 88 or 92 degree arris shaves wood as easily as a dead-on 90.000. Stanley #80-type scraper blades are sharpened to a fairly acute bevel, but work just fine. Sharpening to a square bevel just gives you two extra cutting edges per sharpening.