Varnished wood should be protected with furniture wax. As wax and varnish are perishable, protect surfaces from water, alcohol, other liquids, foods, and cosmetics. Table pads, glass tabletops, and other coverings work best.
Dust regularly with a soft dry cloth. If the finish is waxed, do not use oiled or treated cloths, as they may make the wax sticky. Occasional rubbing the surface with a clean, dry, soft cloth removes smudges and dust and leaves a sheen. Some varnish finishes may be wiped with a moist (not wet) cloth to remove fingerprints and light soil, followed at once by rubbing with a clean dry cloth. Test first on an inconspicuous spot to be sure this does not damage varnish. Do not get varnish wet, or allow a damp cloth to stand on it.
When dirt or grime has built up, clean with a solvent-based furniture cleaner polish, or wax. Use one that gives the desired gloss compatible with varnish gloss, high or low luster. Most polishes and waxes leave a layer of wax on surfaces to protect the finish. When cleaning, do only a small area at a time and wipe dry with a clean cloth. Waxed surfaces may be buffed occasionally to restore shine. Rewax if buffing does not restore a shine.
Excess wax or polish is an enemy. Remove it with a cloth dampened with mineral spirits or synthetic turpentine. (Natural turpentine may leave a sticky residue.) Should the furniture be badly soiled due to neglect, use very fine 3/O or 4/O steel wool instead of a cloth and rub with the grain of the wood. Some finishes can be damaged by prolonged contact with mineral spirits. Clean small areas at a time. Wipe each area with a clean cloth before going on to the next. Discard steel wool as it becomes soiled.
When using mineral spirits, turpentine, or other solvents, including solvent-based cleaners, follow all label warnings. They are flammable, so don't use them near any flame spark, or pilot lights, and don't smoke. Wear rubber gloves to protect your hands, and dispose of them afterward, or wash in hot suds and air dry. Air-dry cloths used in cleaning to evaporate the solvent before disposing of them.
Some varnishes may be washed if badly soiled. Test this method first in an inconspicuous spot; if it streaks, or turns white and hazy, stop. If it's okay to proceed, use a solution of mild detergent (hand safe dishwashing liquid) and lukewarm water. Use a clean sponge or soft cloth. Wash, rinse, and dry only a small area at a time, working fast to avoid over-wetting the finish. Avoid excess water, especially around joints. When completely dry, polish or wax. In most cases, it saves time and energy to clean with furniture polish/wax in the first place.
Oil soaps may clean satisfactorily on some varnish finishes; test first in an inconspicuous spot.
Test for Existing Finish on Furniture
Rub a few drops of boiled linseed oil into the wood. If it absorbs, the wood has an oil finish. If it beads up, the wood has a hard finish (continue).
Rub acetone over a spot in a gentle, circular motion. Polyurethane finishes shed acetone like water. Lacquer dissolves in 30 seconds with rubbing. Varnishes and shellacs turn to a sticky, gel-like substance after a minute or two (continue).
Try a few drops of denatured alcohol. Shellac dissolves quickly in denatured alcohol. Varnish reacts slowly.