ST. LOUIS – The Gateway Arch gradually is losing some of its silver sheen, as a variety of forces — from salt to body oils to graffiti — take a toll on the stainless steel monument to westward expansion and a symbol of St. Louis.

Now, a group of national nonprofits and preservationists are suggesting ways the National Park Service could begin to clean the arch's skin.

The Park Service investigated arch corrosion more than a decade ago and finished a report in 2006. But in 2015, after study — and testing that featured people rappelling down sides of the arch for the first time in its history, in order to gather samples — park officials said it was "not feasible" to pursue a full cleaning.

By 2018, however, several outside organizations, including the Los Angeles art preservation nonprofit the Getty Foundation and the Association for Preservation Technology in Springfield, Ill., had started their own investigation into conserving the arch. The agencies finished their report last August. Arch officials got copies in December and held a conference in February.

The arch has not been fully cleaned since shortly after its completion. After its keystone was installed in the fall of 1965, "final cleaning, repair, and polishing" was done by hand, the new report says. That cleaning process lasted a full year and created some of its own visual inconsistencies: Where stabilization struts had been placed during construction needed particular attention.

The 269-page report documented preservation strategies that the Park Service could use to "clean, possibly refinish, and generally conserve" the structure's stainless steel surface.

The study examined the potential use of such wide-ranging means as drones, lasers and rappelling, in addition to the more conventional use of cleaning agents.

The report builds on a handful of earlier studies completed from 2006 to 2015. The varying analyses detail the list of threats that pose challenges.

At the ground level, de-icing salt has caused "superficial corrosion," the report says. Higher up, studies have pointed to atmospheric pollutants tarnishing the arch's upper reaches. And "many of the visual anomalies" are marks left from the monument's original construction, in which cranes and machinery had to climb the structure and anchor to it while work progressed.

Perspiration, body oils and graffiti — especially harmful when etched into the stainless steel — pose the risk of serious damage "if left unchecked," according to the report.

"It's just things that you wouldn't think of, but they can build up over 56 years of time," said Pam Sanfilippo, a program manager of museum services for Gateway Arch National Park, summarizing many of the arch's exterior's problems. "You would think, 'Doesn't rain just wash it off?' Not necessarily."

Cleaning techniques examined in the report included the use of lasers, a combination of pressurized water, steam and dry ice, and manual polishing with chemical cleaning agents from Astro Pak, a California-based company.

Even to those who participated in the study and interacted with the Park Service, there is no indication that the Park Service intends to clean the arch soon.

"We're not holding our breath for that decision to be made," said Mary Cheng, an Astro Pak spokeswoman.

She said the company came away with the sense that there's not great urgency to address the monument's corrosion challenges, and suggested that the Park Service even could be willing to wait years for technological improvements to present solutions.

Despite the forces at play on the arch, Sanfilippo said, "it's still amazing how reflective it is."