Vertical belt sander is the best. Perfect 90 degrees.
Dress the bar with a fine draw file later , sander leaves 90 degree marks depending on grit used.
Check your groove for width and depth. Never had to trash a Stihl ES bar R + R this way. Some of those bars were free, and fugly!!!
A used vertical belt sander, comes up on CL often , for less than other purpose tools and is more useful.
best thing I have seen is the edge dresser on a Ski/snowboard grinder. you can adjust in 1/2deg increments for the edge grinder...just set it at 90 deg and off you go. as it has liquid cooling pumps that put collant on the item being ground you do not have to worry about overheating the bar either.