For cleaning old shellac and varnish finishes I use a concoction
comprised of equal parts of boiled linseed oil, turpentine, denatured
alcohol, white vinegar.  This not only cleans the surface but
conditions the finish.  It will remove finish if not controlled.

For lacquer and polyester finishes, a dilute solution of "Simple Green"
degreaser/cleaner does a great job.  For very dirty shellac/varnish
finishes, I might try the Simple Green first to remove the grime,
followed by the 'recipe' to further clean and condition the finish.

Jon Page