I was dealing with this not too long ago, but I got frusterated and took a half dull pocket knife to it and it worked wonders. Just stand the stave up on it's end, hold it in one hand and make short, strong strokes with the pocket knife with the blade almost flat on the bark. Once you get the hang of it the bark with come off easily, but you'll be in no risk of ruining the back because the knife is too dull to do that. Seemed to work well for the few staves I tried it on, and they were all cut in the winter.