Shellac and lacquer finishes are not resistant to water and alcohol. Spills and condensation from glasses can leave permanent white spots or rings on these finishes. To remove these white spots, first try polishing the surface with liquid furniture polish; buff the surface firmly. If this doesn't work, lightly wipe the stained surface with denatured alcohol. Use as little alcohol as possible; too much will damage the finish.
If neither polishing nor alcohol treatment removes the white spots, the damaged finish must be treated with abrasives. Gentle abrasives can be purchased from a home-supply store. To make your own gentle abrasive, mix cigarette ashes to a paste with a few drops of vegetable oil, light mineral oil, or linseed oil. Rub the ash-oil paste over the stained area, along the grain of the wood, and then wipe the surface clean with a soft cloth. If necessary, repeat the procedure. Stubborn spots may require several applications. Then wax and polish the entire surface.
If rubbing with ashes is not effective, go over the stained area with a mixture of rottenstone and linseed oil. Mix the rottenstone and oil to a thin paste, and rub the paste gently over the stain, along the grain of the wood. Rottenstone is a fast-cutting abrasive, so rub very carefully. Check the surface frequently to make sure you aren't cutting too deep. As soon as the white spots disappear, stop rubbing and wipe the wood clean with a soft cloth. Then apply two coats of hard furniture wax and buff the wood to a shine.