Hello Andrew!
You must be very careful doing such things! It is very easy to break a knuckle, especially the knuckle of the small finger! Be sure that the impact is distributed evenly among the bottom three knuckles of the vertical fist. I suggest starting with the wing chun wall bag, filled with dried beans. After many months of this type of punching, switch to sand. After many months of punching the sand filled bag, switch to BBs. This is a gradual progression, and is a somewhat safe way to train the knuckles. This will also allow you to work on that "last second thrust" for the bottom three knuckles to drive forcefully into the target! Also, if you like, you might want to purchase some dit da jow to help strengthen the joints and skin for impact. It is available from most traditional gung fu supply shops. It stinks though, so you might not want to go around the ladies until after the smell goes away!

Keep Blasting!
Sifu Lamar M. Davis II
Senior Instructor
Hardcore Jeet Kune Do
Sifu@HardcoreJKD.com
http://www.HardcoreJKD.com
http://www.RealCombatOnline.com
Phone (205) 956-1901
"Hit Hard, Hit Fast, Hit First - ALWAYS!"