Hi there fellow bowyer, Denny here< I would venture to say you have done what is needed in the process. The color is not uncommon among the staves I have worked with over the 23 years I have scraped them.They have never created anything but beauty. As to sap wood used as backing. Why not? it makes a nice white contrast and is a natural backing of the stave . Have done it with lots of success.This is if it isn't too thick, I recommend 1/8 of an inch. I hope this helps. Denny