THEY'RE PUTTING UP an absolutely outlandish exhibit at the National Building Museum. It was supposed to be finished last week, but with any luck it won't be finished next week either, because the process is as interesting as the product.

What's building is a vast and towering monument to the craft of sheet metal working, in celebration of the centennial of the Sheet Metal Workers' International Association. Soaring 65 feet, the structure is meant to demonstrate the skills and ingenuity required to turn flat metal into shapes that are always useful and often beautiful.

The project, which began in mid-December, has been an all-volunteer labor of love by scores of master metalworkers from the United States and Canada, aided by dozens of apprentices (many working at the museum far into the night after a full day on a regular job). Parts of the structure weighing as much as a ton had to be manhandled through the narrow doors, because there's no way to use a crane.

It's easy to see what is inspiring such enthusiasm. Architect Frank Gehry's design is devilishly playful, curving and dipping and soaring and thrusting. The, ah, thing is also nameless, which makes it curiously difficult for the docents to talk about.

While the purpose was to display every possible technique and material used in modern sheet metal work, the result can be distinguished from abstract sculpture only by its usefulness. Inside the two-part structure are galleries of artifacts of this artificer's art from ancient times to the present, along with displays of the tools of the trade, from tinsnips to computers.

The exhibits are interesting and well-presented, but the star of the show is the package they come in. Visitors tend to reel slightly as they walk around the structure, and one of the sheet metal men explained why. "I don't think there's a true vertical or horizontal in this thing except the floor. All the specifications call for 88.5 degrees here, 92.5 degrees there."

"Some of the elements are just in-jokes, like that idiot radius over there," another explained, pointing with the bandaged finger that's the badge of his trade. "If you worked with sheet, you'd know from the specs that it would be just about impossible to do that without getting an ugly ripple. It was meant to be a challenge, and we did it. I tell you, this guy {architect Gehry} is really mean, but we've stayed right with him."

The griping is constant and colorful, but the tone is affectionate, because Gehry, by testing both the metals and the metalworkers' mettle, has given them a splendid chance to stretch themselves and to show off. Normally the ducts and plenum chambers and vents and sheathing they construct are both prosaic and concealed within columns or behind partitions; and you're not likely to notice a metal roof unless it leaks.

The setting, which might overwhelm a less ambitious work, is the great hall of the Old Pension Building, the grandest interior space in town. The century-old structure is in the midst of restoration, but the museum staff hopes to open at least part of one of the encircling balconies so visitors can get an overall perspective.

Although only a seven-month run is scheduled for the exhibit, it's to be hoped they'll come to their senses before the demolition crew moves in. Or perhaps not; it might be just as interesting to watch them take it apart, so long as they save the pieces to be reassembled somewhere else. THE GREAT BIG BEAUTIFUL METAL THING -- Through August at the National Building Museum on Judiciary Square (Metro Red Line). Open noon to 4 weekends, 10 to 4 weekdays. Free.