If you have wooden painted stairs, you will not be able to stain them. Paint is durable and inexpensive, and you can change the color depending upon your decor. Once stairs have been painted, the only way to remove the paint is to sand it off or use a liquid paint stripper. No other product—even stain—should be applied over a painted step as it won't adhere to the paint. In general, if stain is applied over paint, it will remain sticky and only partially dry, which will require some major work to remedy. There are some latex stains that you can apply over paint to darken the surface, but you won't achieve a wood grain look with this application.