If it's a really large area, you might want to rent a drywall sander.
Porter Cable makes one - it's a motorized rotary sander using about
12" disks mounted at the end of a lightweight tube which is then
connected to a shop vacuum. Using 80 grit disks, you should be able
to take off all the high spots. If there are low, then just go over
them with a few coats of joint compound, sanding after at least the
final coat and paint.
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