I've read of people doing all sap wood, all heartwood and a mixture of both for making black walnut bows. The staves I have now- as is- would basically give me an all sap wood bow with some heartwood coming into the handle area. I would like to do a very thin sap layer on the back with most of the limbs being heartwood, but can not see any discernible growth rings on the sap wood. Should I just thin it where I want it and back with rawhide? Do I need to have the sap wood at one growth ring? Any help is appreciated. Thanks!