Many years ago my TaeKwon-do teacher taught us how to harden our knuckles.

1. Punch the heavy-bag (canvas) until your knuckles bleed.

2. Apply tincture of benzoin. (sticky stuff, smells like pine.

3. Scream really loud.

4. Punch the heavy bag some more.

5. Scream really loud some more.

6. Repeat steps 1 through 5.

My knuckles stayed calloused for about fifteen years. I thought most of the nerve endings were dead, but some of the feeling has come back recently.