IF the piece that you want to Varnish is CURRENTLY STAINED AND VARNISHED then you should give it a Light Wiping with Lacquer Thinner and a Light Sanding with Fine Sandpaper, then dust it off, then Varnish it...

     IF the piece is CURRENTLY PAINTED, then you can apply a protective coat of Varnish over it directly, if it is free from dirt, grease or wax... however this might cause a slight "yellowing" of the surface color...