DOWN IN the bowels of the National Collection of Fine Arts, 8th and G Streets NW, is one of the most exciting shows in the whole museum. Yet it is not on the public itinerary.

Here, off a long hallway lined with crated art works, amidst a certain amount of stored junk and in an atmosphere something like that of a warehouse, two artists quietly turn out masterpieces. They're virtually unnoticed, except by people who know.

The artists are Oliver Anderson and William Adair. They are the principal picture-frame restorers for the National Collection and the National Portrait Gallery, respectively.

One of Adair's recent works was the restoration of the gilded 19th-century French armchairs, once owned by President Monroe, in the White House entrace hall. He has also worked on several frames in the White House -- some after the serew-eyes holding them up had torn out and the paintings fallen off the walls in the night. And he is scheduled to work on the eagle console table in the State Dining Room and some French Empire card talbes.

Heady stuff, indeed.

Adair is becoming perhaps the local master in the art of gold left gilding. If that sounds like the most obscure thing you ever heard of, take a walk through the collection -- or any number of important museums -- and be amazed at how gilding proliferates in art. Not just the art of years gone by. But today's art as well.

Gilding -- on icons, murals, sculpture, elaborate frames, furniture and even buildings -- remains an important decorative tool. Whether it is always done properly is of considerable question. Often, it is not.

A walk through more than one national art gallery, for instance, reveals, even to the beginner, gilded sculptures and frames touched up with such substitutes as bronze powder, "radiator paint," gold spray paints and "Dutch Gold," or gold-colored metal leaf.

These produce what Adair calls "The Green Grunge." Unlike gold, which never tarnishes, substitutes, which are merely metallic pigments containing no gold, turn different shades of green or black.

To gildsmen, who regard 23-karat gold leaf as befitting a valuable work of art or antique, the others are an abomination. And to mix them sheer horror. Sometimes, says Adair, frames otherwise worth thousands of dollars have had "radiator paint" applied completely over the original gilded surface.

This can seriously lower the value of the frame.

To restore such a frame, the "radiator point" must be removed -- which, in some cases, also removes the gilding beneath it. The original finish is lost.

"It is most definitely a national problem," says Adair. "It is a pressing problem. We should make people aware of this so things are not further destroyed or their value lessened. Especially since there are so few gilders left."

Adair graduated with an art degree from the University of Maryland in 1972. He has worked with Anderson ever since and recently, after learning most of what Anderson could teach him, he traveled for about three months, visiting more than 20 shops in Switzerland, England, Boston, Heydenryk's in New York and Italy -- where gilding is an industry, he says.

He hopes to be out with a book on the subject within the year. A few Sundays ago, he shared his knowledge with novices at one of the Smithsonian Associates' many workshops.

Adair says he knows of only one local shop -- Mickelson's, on G Street. NW -- commercially gilding frames with real gold leaf. One reason is the cost. An average size frame can run $300- $400, at least. Unless the frame is of some value, gilding it just isn't worth the expense.

This leaves most of us in the lurch. Fortunately, gilding can be done at home, although the process consumes much time and the possible conplications boggle the mind.

The materials consist of 23-karat gold leaf, which comes in packets of 25 that cost about $15, a varnish called "gold size," water, alcohol, a plaster-like compound called "gesso," a refined colored clay known as "bole," a gilder's tip, or brush, rabbit-skin glue, special burnishing tools made of agate, cotton, paint brushes, casein paint and a book on how to do it.

Gold leaf, which is about one-250,000th of an inch thick, can be applied to almost anything. The technique is essentially the same for furniture or picture frames. The more experienced gilders pick it up out of the packet with the gilder's tip.

"If there's low humidity," Adair said, "it can be extremely difficult to handle because of the static electricity in the air."

Novices can buy patent leaf, which goes on like a decal by rubbing the back of the paper to which it is adhered.

The basic steps for gilding on raw wood read something like a recipe. Start by sanding down the wood, getting rid of any irregularities. Full blemishes with thickened gesso putty. Then give the wood a wash of rabbit-skin glue. This helps subsequent coats of gesso adhere. Allow the glue to dry. It should have a satin finish before applying gesso.

Gesso (Adair recommends the kind made by Grumbacher) is used hot, but not boiling. It goes over the entire wood surface in thin coats. It can be sanded after the second or third coat with fine grit sandpaper, making sure to remove all dust before final coats. You can put on as many as five or six coats, although three or four is usually adequate. The more gesso you put on first, the more brilliant will be the gold after. Every imperfection in the surface will be exaggerated when the gold is applied.

Next apply the bole -- also hot -- with a sable brush over the gesso.Here the gilder faces a choice. Because gold leaf is somewhat translucent, it picks up the colors behind it. Yellow bole makes breaks in the gold less obvious. Red Gives a warm tone. Blue the most brilliant tones. Adair recommends a complete coating of yellow, then discriminating use of red and blue.

Sand with extra fine sandpaper to remove brush marks. Remove all dust before applying the leaf.

Here you face a second choice. You can put on the leaf with either a water size or an oil size ("gold size"). The first is a water-alcohol mixture, also known as "gilder's liquor" (nine parts water, oen part alcohol) you mix yourself; the latter a varnish-like glue. The water technique gives the most brilliance.

If you go with oil size, first coat the surface with a weak shellac solution, not leaving any brush marks.

There are two types of oil size -- a slow-dry and a quick-dry. Adair recommends the quick-dry because you cannot "tone down" (coat the gold with a "casein" paint) later with slow-dry. "Slow-dry never really dries at all," Adair said. The casein, when it dries, can therefore cause tensions that may crack the gild.

The gold leaf goes down immediately after the water size, or when the oil size has reached the proper "tack." (You can use the knuckle test with oil size. Place a knuckle on it. It should come up without any of the size, but still making that "click" sound.)

Adair uses the regular, loose leaf. He picks it up with the broad gilder's tip after drawing the brush lightly across the back of his hand, where he has rubbed some Vaseline Intensive Care lotion. He cuts the leaf into the size pieces he wants with a "gilder's knife." An artist's palette knife can be substituted.

Apply the lear, tamping it down with the cotton, which must not touch any of the size, lest it stick and foul up things.

If you use water size, the gold can be burnished to the sheen you want after it is laid. This is done with a burnishing tool that has an agate tip (it can also be done with a dog's tooth), simply rubbing the tool ove the gold.

Gold applied on oil size cannot be burnished. It can be buffed with cotton, however.

If you are not satisfied with a glossy finish, you can tone the gold down with casein. First put a protective coating of shellac over the gold. But remember, slow-dry size will not take it.

More advanced gilders can try water size and oil size techniques together -- putting leaf on with water for highlighted areas, then oil size and leaf over that. Repairs can be made as well. If some pieces have fallen off or broken, for instance, you can replace them by making casts elsewhere on the object using clay and a plaster called "Roma Plastilina" (No. 2 or 3).

And there you have it. Or maybe not. If you're still wondering where to get all these things, try one of the larger artist's supply stores or hobby craft shops. If they don't have it, they should be able to order it.

And if this description has you a bit confused, Adair suggests reading: The Practice of Tempra Painting , Daniel Thompson, Jr. (Dover Press) or How to Gild Antiques and Other Art Objects , Donald Chambers (Crown Publishers).