If the dataplates are of the raised letters type, ive had success with
just removing all paint, including the black background with paint
remover. Then repaint the sign, and use steel wool or a fine knife to
remove paint on the raised letters, and finally a top coat of clear
paint. This is takes some time, but gives excellent results. If you
dont want to do this, or if the letters are not raised, I would just
carefully remove the laquer with fine steel wool, and then add a new
clear coat.
Lars Svaasand