Poly-U, Tung Oil, Spar Urethane are the ones that I've used. All with good results. If brushing on, I've used a cheap sponge brush. First I dipped the brush in thinner and let most all of it run out. Then dipped it in the finish to brush on. Otherwise, what I do today is, fill a PVC tube with finish. (The initial investment is a bit.) Then I dip half the bow at a time in the tube, let the excess run back into the tube then hang to dry with newspapers or something underneath so as not to ruin my floor. When the first dip is dry, usually overnight, then I dip the other half of the bow. I usually dip 3-4 times total each limb with snake skins on.
Be sure to remove the scales from the snakeskin when it is dried prior to dipping for the first time. Good Luck!