I used a 4" grinder, but for the corner stuff, go with a die grinder with a small diameter burr. Something like THIS.
Robert (username MPandC) would probably say use a small cut-off wheel on edge till you get close to flat, then go with a flap disk on a 4" grinder.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.