Knuckle Pushups might harden your knuckles, but they will not make you ignore the pain you get from hitting boards.
Personally i have a small board, wrapped completely with thin rope, i can use it while sitting near my computer using one hand holding it, and one hand hitting. Or hang it on a wall, for full workout ..
This might bleed the first few times, but only untill you grow stronger skin .. and this way you also learn to ignore the pain of hitting.
__________________
-+-+-+-+-+-+-+-+
Subform - Allover the place
|