In 2005, the park hired consultants for the first of four phases to tackle the problem. Two consultants examined construction documents and Arch leg interiors, took photos of the stains, interviewed a “key Arch project engineer,” and concluded that the stainless steel skin needed to be directly examined. The first phase cost $60,000.

The second phase, in 2010, cost about $330,000. Workers bored samples of the Arch’s interior structure, removed concrete for analysis, and installed temperature and humidity monitors inside the legs, among other things. That work found that “no moisture and very little corrosion” had gotten between the stainless steel skin and carbon steel innards, according to a park service summary of the report.

This phase, Honious said, should identify the cause of the staining and corrosion.

The work, being performed by Chicago contractor Wiss, Janney, Elstner Associates, will cost about $340,000. The six-man team involved in the climbing had been scheduled to finish work here this week, but Monday and Tuesday were too rainy and Wednesday was too windy.

Thursday dawned sunny and temperate. Wiss project leader Stephen Kelley said his group, dubbed the “difficult access team,” were on top of the Arch by about 7:30 a.m.