Don't get caught in the trap of thinking you get a mirror polish by buffing - you don't.
99 and 44/100% of the job is sanding!
Guys try and try to use buffers to remove scratches and that's a waste of time.
Remove scratches with finer sandpaper!
Good methods were shown above. Personally I go 220-400-800-1200-1500. Paying close attention to make sure all previous scratches were removed from the preceeding grit before moving on.
Buffing should be just a quick Wham-Bam! and go right to mirror polish.
But, like everyone else said, one tiny little blemish on a perfectly executed mirror polish and it's toast!
A better finish, for a utility knife, would be to take the blade up to and fully through
600 or 800, pulling the stroke the entire length of the blade. Nice flat finish and fine scratches won't ruin your day!