two choices
Michael Dresdner
>Assuming they have a finish on them, you can do one of two things. One option is to strip the finish with paint remover, get back to the bare wood, restain and apply a new finish. This is a tedious and messy project.
The other option is to darken them with toner. Technically speaking, you can not stain very successfully over an existing finish, but you can glaze or tone. Glaze is thickened stain floating between coats of finish. While it is great for some things, for what you describe toner would be better. Toner is simply tinted finish. By adding coats of toner, you add both finish and color. You can find toner in both aerosol cans, at any woodworking specialty store, or as tinted brush on finish in any home store.
To add toner, first clean the surface with a good degreaser, such as mineral spirits or naphtha using fine nylon abrasive pads. That will dislodge dirt and wax while simultaneously lightly abrading the surface. Seal the piece with one coat of Zinsser SealCoat, then have at it with whatever toner you have chosen. Bear in mind that if you apply toner unevely, you will end up with uneven color, so practice first on scrap.