There was a time when I barnished the back of white wood bows. Where it's hard to follow a ring (HICKORY) I used a beer bottle, Pappy right stain it first and barnish it then I still wool the barnishing will make it glossy. If you don't finishthe bow first you have to paint it. I had a friend that barnished the whole bow if it was white wood. Never did under stand barnishing the belly.
When you barnish it,it compress's the wood cell helps to stop splinters raiseing. Will help on short heavy bows or bows that have some sort of defect in the back. And alterent for backing.