On any given day, the preservation work done at the Smithsonian National Postal Museum is miniscule — literally. Museum staff work to preserve stamps, the key items in most of the exhibits, in small rooms of the historical City Post Office Building on the corner of Massachusetts Avenue and North Capitol Street Northwest.

But once a year, a much bigger task is at hand. The large exhibits that fill the museum’s atrium, including planes that hang from the 90-foot-high glass ceiling, require a thorough cleaning that can only be done after hours so as not to disrupt visitors from enjoying the exhibit.

From Aug. 23 to 25, museum staffers rotated shifts and stayed from 5:30 p.m., when the building closed, to 2 a.m. to supervise and participate in a process that included dusting and shining the vehicles on display.

In a painstaking process, the contractors move a lift forward foot by foot. One person places a 4-by-8 wood plank in front of the wheels so the lift won’t scratch the marble floor. Another person drives the lift forward. Then it repeats — move wood, drive forward, move wood — until they’ve reached the desired spot.

It’s a slow process, said Linda Edquist, conservator and head of the museum’s preservation department. But it’s a necessity.

Imagine the buildup of dust in your home. Now multiply that times 400,000, and you’ve got the amount of dust that builds up annually in the atrium with that many visitors passing through.

“We need to get into the nooks and crannies and do a thorough job,” Edquist said.

The cleaning is more about preserving pieces of history than about aesthetics. One of the planes, the Wiseman-Cooke, was used to carry letters in California from Petaluma to Santa Rosa in the early 1900s. The plane crash-landed in 1911, but it was repaired and used for a trip in India later that year.

Another plane, the de Havilland DH-4, was used while the Post Office Department was creating its first national airmail service from 1918 to 1926.

A third plane hanging in the atrium, the Stinson Reliant, was used in 1939 to test airmail services for places that didn’t have landing fields in Ohio, New York, Pennsylvania, Kentucky and West Virginia.

“If we didn’t do [the cleaning], it would build up to the point where the joints, hardware and plastic would be affected, tarnishing these objects we’ve worked so hard to preserve over the years,” Eric Chapman, the museum’s exhibits specialist, said as the lift’s beeping echoed through the building

Chapman rode in the lift one afternoon, helping clean the objects with solution and replacing light bulbs in the entrance.

When the museum first opened in its location at the old City Post Office Building in 1993, the National Air and Space Museum took care of the cleaning because the objects are actually on loan from there. But after more than a decade of handling the task, it was decided that the Postal Museum’s preservation staff would take over, simply because it was easier for the staff to organize the tasks.

From below, the cleaning isn’t that obvious. The gleam of the shining objects can only be observed from the offices that face inward into the atrium, including the Bureau of Labor Statistics and Senate office space on the sixth floor.

“They tell us how much they appreciate the work when we’re done,” said Meradyth Moore, the museum’s public information officer. “I can only imagine what it’s like to see the dust one day and a shining plane the next.”