I would gesso the back and lightly sand till smooth
draw with a pencil,,
if you don't like it,, erase or lightly gesso over it and try again
if you want an ochre tone,, put ochre in the gesso,, or any color you want the background to be,,
then finish over with any varnish or spray that is compatible with the acrylic,, since it is sinew,, I would then put a heavy layer of poly over that,,in addition to what you sealed the acrylic with..
if there is a glaze of hide glue,,,, then use a test piece of wood with the same glaze to see what works best to seal that,,,,
once sealed,, then poly over that,,
archival art varnish will seal most anything without damaging it,,then you can poly over that,,
but if in doubt ,, always best to try on a test piece,,