CAPE CANAVERAL, Fla. >> A spacewalking astronaut ran into trouble Sunday while trying to lubricate a joint in the life-sustaining solar power system of the International Space Station, losing one bolt and getting a washer stuck in a crevice.

Mission Control told veteran space flier Mike Fincke that he’d have to settle for a partial lube job, after the bolts holding down covers on the joint started popping off unexpectedly.

“Bummer,” said his spacewalking partner, Andrew Feustel.

The spacewalk — the second of shuttle Endeavour’s final space station visit — started out well, with Fincke and Feustel topping off a leaky radiator line. They successfully added ammonia to the space station’s coolant system, after rerouting jumper cables and opening valves. One line leaks slightly, and the astronauts needed to recharge it.

Ammonia is extremely hazardous, and the two did their best to avoid contaminating their spacesuits while replenishing the system with about 5 pounds of the substance. Some frozen ammonia flakes floated toward Feustel as well as a small icy chunk, but he didn’t think any of it got him.

Fincke moved on to preventive maintenance on the massive joint that rotates the space station’s solar wings on the left side. He was removing his first cover when a bolt popped out and got away from him. He caught it. But another bolt ended up floating away, and a washer got stuck between the cover and an attachment.

Mission Control worried the washer might get into the gear mechanisms of the joint and damage them. Fincke was advised to use “gentle backhand sweeping motions” to get the washer away from the gears, and the astronaut replied he could try to coax it out.

All this came as a surprise, and Mission Control later told the astronauts that the washers might be bent and flimsy from previous repair efforts.

“Sorry you’re having such a hard time with those bolts, buddy,” Feustel called out to Fincke.

“Yeah, man, I was being really careful, too,” replied Fincke, one of NASA’s most experienced spacemen.

After some deliberating, flight controllers instructed Fincke to remove only four covers from the joint rather than six. “We’ll do the best job we can,” Mission Control radioed.

Another bolt popped out and almost got away, but Fincke caught it. “He gets the golden glove award for another catch,” Mission Control said.

To Fincke’s relief, the remaining covers came off much more easily.

Fincke and Feustel teamed up for the actual lubrication of the joint, using grease guns to squirt the dark substance onto the gears. They turned to other tasks as the joint was rotated to spread the grease.

Mission Control asked the astronauts whether they were up for more work on the joint once the motion stopped. They said they were feeling fine and wanted to keep going. The spacewalk, originally set to last 6 1/2 hours, was expected to go into an hour of overtime.

NASA wants to lubricate as much of the joint as possible to keep it functioning properly in the years ahead without any shuttle visits.

A similar joint on the opposite side of the orbiting lab became clogged with metal shavings in 2007 because of grinding parts; it took a series of spacewalks in 2008 to fix it. During one of those excursions, a tool bag got away from an astronaut and floated away — it was a $100,000 loss.

The circular joints — 10 feet in diameter — turn the space station’s solar wings toward the sun, like the paddle wheels of a boat. Each set of wings measures 240 feet from tip to tip. The panels collect sunlight and convert it into electrical power that’s used to run all the equipment aboard the outpost, including the life support.

These are the final spacewalks to be conducted by visiting shuttle crew members; the goal is to leave the space station in the best possible condition for its next decade of operation. This is the next-to-last shuttle mission. The 30-year program will end in July with the flight of Atlantis.

As Sunday’s spacewalk got under way in the wee hours of the morning, astronaut Gregory Chamitoff observed from inside, “This is an important one for the longevity of the station, for the power and cooling.” He added: “So let’s get started.”

As they ventured out more than 200 miles up, Fincke and Foale praised one another. Both are experienced spacewalkers; this was Fincke’s seventh outing and Feustel’s fifth.

“It’s an honor to be walking, spacewalking with a Hubble spacewalker,” Fincke told Feustel, part of the 2009 Hubble repair team.

“It’s an honor to be walking with the man with the most time in space,” Feustel replied. Fincke will become the most traveled American in space by next weekend, surpassing the current record of 377 days aloft.

Feustel and Chamitoff had to cut Friday’s spacewalk a little short because of a failed carbon dioxide sensor in Chamitoff’s suit. That problem wasn’t expected during Sunday’s outing.

On Monday, three of the six space station residents will head home in their Russian Soyuz capsule after a five-month mission. In a unique photo op, the departing crew will photograph Endeavour parked at the space station.

Then on Wednesday, Fincke and Feustel will venture back out for spacewalk No. 3.

Endeavour, under the command of Mark Kelly, husband of wounded Rep. Gabrielle Giffords, will remain at the orbiting outpost for another week. Landing is scheduled for June 1.