To refinish an old upright piano, disassemble the instrument into pieces, strip off the old finish, sand each piece, apply stain and put on a finishing coat. When pieces are completely dry, reassemble the piano.
When disassembling the piano, keep a record of the order in which you remove the pieces, using notations such as "left" and "right" where appropriate, to aid in reassembly. Work from the top of the piano down, removing the lid, top front panel and side rails. Move on to the areas around the keyboard, removing the music shelf, the keyslip, the fallboard and the keyblocks. Finally, remove the bottom front panel and any decorative plates over the pedals.
Lay out the pieces, and apply a commercial paint and varnish stripper, typically designated as a 5F5. Remove the old finish using a knife or putty scraper, applying additional coats of stripper as required to get down to bare wood. Wash down the wood with a solution of equal parts lacquer thinner and alcohol.
Allow the pieces to dry completely, and sand all surfaces with 100 to 150 grit sandpaper. Brush and vacuum any dust, and then apply stain to all components. Finish all pieces with multiple coats of polyurethane, allowing each to dry completely before applying a new coat. Reassemble the piano, referring to notes.