The problem with most hard spots is that they're football-shaped, with most of the hardened metal below the surface. It's usually necessary to do a VERY deep cut to get rid of them. Simply grinding a hard spot flat doesn't remove it- - - -the metal around it will wear away and the high, hard area comes back and causes the clutch disc to wear and chatter.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!