Extending the life of your furniture

''Furniture is so costly these days that we want to know how to take care of it so it will last a very long time.''

That's what Tom Benson, furniture-care specialist for Johnson Wax, now hears most often from consumers around the country.

Many complaints consumers lodge against furniture manufacturers or care-product manufacturers are not the fault of either, Mr. Benson points out. They have to do with the natural enemies--heat, humidity, and sunlight--which can be hazardous to furniture unless they are controlled. He gives this advice:

* Since constant exposure to direct sunlight can cause crazing, checking, and fading of the finish, arrange furniture away from sunlight and rotate it whenever possible. Remember that any object left on a surface in bright sunlight , such as a lamp or vase, will in time leave its mark.

* Since wood furniture can be affected by severe temperature changes between hot and cold, don't place it next to a radiator or other heat source, or anywhere (such as in front of a window) where it would have to endure cold blasts or sudden temperature drops.

* Since extremes of humidity can warp or crack wood, damaging its appearance and even its function, use room or central dehumidifiers in humid climates and humidifiers in extremely dry regions. Or remember that the old-fashioned practice of placing pans of water next to heat sources is still a simple remedy for increasing humidity.

As for applied care, Mr. Benson says the purpose of polish is to protect furniture finish rather than the wood itself. Many homemakers insist they must ''feed'' the wood. But most furniture manufactured in the United States today (over 80 percent) is thoroughly sealed with a lacquer finish and cannot be ''fed.''

Oil-based products are available for oiled finishes that have not been sealed with lacquer. Much of the teak and oak furniture produced in Scandinavia has such an oiled finish and requires treatment with an oil-based (not a wax-based) care product. If a wax polish is used on an oiled finish, it can usually be removed by rubbing with a cloth, lightly dampened with odorless mineral spirits, being careful not to remove the stain.

Any quality wax-based furniture polish, he explains, will remove common household dirt and soil from the now more common lacquer finish. It will speed dusting, leave the surface free of smudges and fingerprints, add luster, and act as a temporary barrier to spilled liquids. Today, the best of such polishes cannot leave residues or build up, he says, because they contain cleaners and solvents designed to remove the previous coat of polish as they are applied.

The polish one selects should depend upon the method of application and the amount of shine you prefer. Aerosols, liquids, and paste wax are each formulated to perform differently and to meet specific consumer needs. Since furniture is manufactured today with high-, medium-, and low-luster finishes, homemakers must know which they have and which products are best suited to produce the gloss they prefer. Paste wax is still the traditional product for care of low-luster finishes and antiques, he says.

Minute scratches, Mr. Benson continues, can usually be eliminated by rubbing the surface with OOOO grade steel wool that is saturated with paste wax. To remove white water rings, he recommends rubbing them with a non-linty cloth moistened with camphorated oil.

To remove candle wax that has fallen onto the table, he advises leaving it a day. Then apply ice cubes held in a tea towel and remove with a plastic credit card or wooden spatula (never a fingernail). Rubbing after a light application of polish will remove any remaining candle wax.