While everyone's been going gaga over the grandiose East Building of the National Gallery of Art, workmen have been doing wonders with the ground floor of the grand old original building.

Fully open to the public this weekend for the first time in 12 years, the ground floor contains 75,000 square feet of exhibition space, nearly half of it created by shifting some of the Gallery's main supports and machinery. They've bored right through the long axis of the building, creating a labyrinth of display areas that finally do justice to treasures long in storage.

The project, largely underwritten by the Mellon Foundation and family, also brings order to what had been a chaotic jumble of administrative offices and the all-important conservation laboratories, which are charged with restoring and preserving such marvels as "The Triumph of Christ," a 500-year-old tapestry that seems almost as fresh and rich as though the medieval artists wove it last week. "We just cleaned it," beamed textile conservator Joseph V. Columbus, calling a visitor's attention to the extraordinarily dense weave of 22 warps to the inch. "It wasn't until afterward that we realized how many silver threads there are in it. See how they gleam now."

How does one go about polishing silver thread without staining the fabric?

"Table salt and aluminum foil," Columbus said. "The salt makes an electrolyte and the aluminum foil, being the more active metal, draws the oxide away from the silver. We thought it a rather neat trick."

Other neat tricks are to be found throughout the new spaces. One of the best is the decision to group most collections by chronology and/or geography rather than genre, so that contemporary masterworks in many media can be compared and contrasted. It is only a step, for instance, from the grand medieval hall of tapestries and furniture to a side gallery holding two stained-glass windows made at about the same time for a church in Florence.

A conspicuous and entirely appropriate exception to the non-genre grouping is the central hall of American naive paintings -- a term preferred over "primitive" by donors Edgar and Bernice Chrysler Garbisch. Sixty-two of the more than 300 in the collection have been hung, and the effect is so overwhelming that it's wise to retreat to the adjacent cafe court to cool out over a glass of wine before marching on into the interior galleries.

The paintings speak for themselves, and of America in the making, but stop in at the museum shop and buy the brief and beautiful exhibit catalogue by Laurie Weitzenkorn; it's an excellent introduction to this far-from-"folk" art, and a fine memento.

Another way to attack the ground floor is to come in by the Seventh Street (west) entrance, now open for the first time, and warm up on the "Drawings from the Holy Roman Empire." This is the first of what are to be rotating exhibitions of works on paper, including many from the Gallery's vast collection that haven't been seen since the mind of man runneth not to the contrary. It's about time.

The sturdy of sight may then undertake the gallery of 170 photographs by Alfred Stieglitz and then press on to the naive paintings, but others may find that leads to sensory-input overload, and decide to save one or the other for another day.

Abandon all hope of escaping before closing time, ye who enter by Seventh Street and turn left. First comes the Widener collection of Chinese polychrome porcelains, 176 pieces that haven't been seen since 1971, and then the captivating medieval hall. From there on, there's what seems like half a mile of sculpture galleries, leavened with decorative arts such as 18 pieces of 18th- century French furniture and the chalice of the Abbot Suger of Saint Denis, c. 1140, used as a communion cup for French monarchs through the Revolution. The last queenly lips to drink from it belonged to Marie Antoinette, who might have kept her head if she had learned to keep her mouth shut.

The new ground floor will need some getting used to; the arrangement of some of the new spaces is curious and somewhat cramped, and will seem more so when crowded. But it seems to have been a matter of choosing between allowing more room or showing more objects, and it's hard to argue with the decision. NATIONAL GALLERY WEST BUILDING -- Fourth and Constitution NW. Open 10 to 5 Monday through Saturday; Sunday, noon to 9..