The only way I know how to do it would be to strip the entire finish off and re-stain/oil it. You can use Easy Off oven cleaner. Spray it on good and thick, let it sit a few minutes and rinse with water. Repeat a couple of times until you get enough of it off. Then use mild soap and soft brush with warm water. Rinse well. Pat it dry and hang to dry thouroughly. Once completely dry, sand with fine grit sand paper. I've gotten some of mine to look like they've never been finished. Then you can stain it, or rub with linseed oil to get the look you want. It's not hard. There are more detailed instructions out there on the web. I've never had it go bad on me. Just don't get the Easy Off on your skin. Very much bad.