A 1" impact wrench works pretty good, like the ones used on semi wheels.

Lacking that a cutting torch putting some heat on the nut while avoiding the bolt also will break a lot of them loose.

I often touch up blades while on the mower but if they need a lot of grinding take them off.

A good welding shop with a forge can hammer them out also which retains more metal than grinding.

I have never hard surfaced any but my Bad Boy Outlaw XP zero turn mower has hard surfaced blades (hard surface on the bottom side) so I would assume it could be done to bush hog blades also.

John

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