The hot lick for sharpening any mower blade is a 5/8 dia. X 1" long solid carbide double cut burr. Chuck that baby up in yer 1/4" die grinder and you wont have to worry about burning the edge, even on the thinnest harry homeowner blades!
Plus, ya can sharpen most in situ!
Pricey, but it will last forever. ( if ya don't drop it, like I did! )