Stain can only be applied to a surface that has NO Paint or Varnish on it and is free of dirt, grease, wax or sealers... So you CANNOT Stain over Paint or over a Stained and Varnished surface...

     IF you want to Stain over a Stained and Varnished OR Painted surface you must first REMOVE all the old Stain and Varnish OR Paint with a Stripper Product, or a Furniture Refinisher Product, following label directions...