I used this technique to lap in a 3 jaw that had worn jaw tips but was
a good chuck otherwise. It worked very very well. I had previously tried
to bore the jaws as was suggested here and the boring bar would not touch
the chuck jaws.

I recommend using the largest brass round stock that you can get into
the chuck because your jaws will be curved not flat. When you grip
something larger than your brass rod, it will be grabbing on the edges,
not in the middle of the jaw.

Also run the spindle as slow as possible to keep from throwing grinding
compound all over. I think I used a fairly course grit, probably 180.
Can't remember if I also used a fine grit afterwards.

chuck