Hi,
I think, if you apply shoe polish (which is basically wax), the polyurathane, or most other finishes, won't stick. I tried the same thing with a leather rifle sling once, and it turned out horrible.
Sand your rawhide with a very fine paper, and then finish with steel wool. After it hardens (which usually takes more than a week), rawhide is a lot like wood. It's worked good for me. I usually cover my rawhide backings with a nice pair of matched snakeskins.