Two spacewalking astronauts armed with caulking guns, putty knives and foam brushes practiced fixing deliberately damaged shuttle heat shields Saturday, a job they hope they won't have to do for real.

Although Discovery (search) suffered some scrapes and chips during liftoff, none of the damage appears to warrant orbital repairs, space agency officials said.

As the two astronauts completed the mission's first spacewalk, NASA (search) was on the verge of extending Discovery's visit at the international space station. With future shuttle flights grounded because of Discovery's fuel-tank foam loss during liftoff, the space agency planned to keep the crew there an extra day to haul over surplus supplies and chip in with some station maintenance.

In a pair of tests designed in the wake of the 2003 Columbia tragedy, astronauts Soichi Noguchi (search) and Stephen Robinson (search) worked on custom-made samples of thermal tile and panels that were cracked and gouged before flight. They squeezed and dabbed dark goo into the crevices as they sped around the planet.

The sticky material got on their gloves and clumped to the ends of their putty knives. But spacewalk managers had feared a much bigger mess and were pleased with the relative neatness of it all.

"It's about like pizza dough, like licorice-flavored pizza dough," Robinson said as the near-black filler material oozed from his high-tech caulking gun. He used a putty knife to smooth down the substance, again and again.

"The cleaner it is, the better work you do, just like anything," Robinson said, holding out his knife for Noguchi to wipe.

The astronauts reported some bubbling in the two repair materials — a paintlike substance for the thermal tiles that cover most of a shuttle, and a thick paste for the reinforced carbon panels that line the wings and nose cap. The paste swelled up in the cracks like rising dough and, as the experiment wore on, was harder to get to stick because of colder than desired temperatures outside.

It was all valuable feedback; engineers wanted to see how their creations fared in the weightlessness of space for possible future use in an emergency. Neither the bubbling nor swelling was surprising, said Cindy Begley, the lead spacewalk officer.

Columbia's astronauts had no such tools or techniques at their disposal. Of course, neither they nor flight controllers knew Columbia had a gaping hole in the left wing, left there by a 1.67-pound chunk of fuel-tank foam insulation that broke loose at launch.

A piece of foam just over half that size came off Discovery's external fuel tank during last week's liftoff. It missed Discovery, but was enough to ground all future shuttle flights. A smaller foam fragment may have struck the right wing, but lasers and other sensors found no evidence of damage.

None of the repair kits flying on Discovery could mend a hole the size of the one responsible for Columbia's catastrophic re-entry, estimated between 6 and 10 inches across. It could be years before engineers come up with such a big patch. For now, the largest hole that any of the repair methods aboard Discovery could tackle would be 4 inches.

The astronauts will test a third repair technique, essentially a plug, inside Discovery later this week.

Once the repaired samples are back on Earth, engineers will analyze them to see how deep and how well the filler material penetrated. None will be torched, however, to simulate the searing heat of re-entry. The spacewalkers had to skip the one sample intended for laboratory test-firing because they ran out of time.

In the first of three spacewalks planned for this mission, Noguchi and Robinson also made some long-overdue space station repairs. They restored power to a gyroscope that stopped working four months ago and replaced a broken Global Positioning System (search) antenna.

"Great job. Everything was just perfect. Extra stuff got done," Mission Control (search) radioed as the seven-hour spacewalk came to a close. "You guys get some rest."

As soon as Robinson and Noguchi were back inside, their shuttle crewmates pulled out their 100-foot, laser-tipped inspection crane to survey Discovery's left wing one more time. Engineers wanted to make sure they didn't miss any signs of damage.

On Sunday, NASA expects to wrap up all its analysis of Discovery's thermal shielding and give the final safety clearance for the shuttle's descent in another week.