All of my spanking paddles are sanded to a very smooth surface and then finished with Danish oil, which is a combination of oils to make it look good and some polyurethane added for toughness. I give all spanking paddles at least three finish coats, with some receiving several more to enhance their look and feel. Some spanking paddles are also given a wax coat depending on the type of wood.
I occasionally use pure tung oil as a finish, which is a slow drying but relatively hard oil finish, which gives a matt finish.
A small number of spanking paddles have received a polyurethane varnish finish, but consensus is varnish doesn’t look nearly as attractive as an oil finish, and most people consider the added protection provided by varnish as unnecessary for a spanking paddle.
Overall, oil finishes look better and are easier to touch up and repair than varnish, but varnishes are harder, and provide greater protection against scratches and stains.