If your skin is rough and already has some finish on it you can sand it lightly with some 600 grit paper. By lightly I mean not more than the weight of the paper. I go back and forth, add a coat of tru-oil let it dry, sand a little more add more tru-oil and so forth until I have an almost smooth skin.
Be very careful on thin skins like copperheads, you can sand through the pattern in a heartbeat.