Here's an article that talks about how the National Gallery of Art does painting conservation - "Virgin Rebirth: Technology Helps Humans Work Miracles on a Renaissance Treasure," By Jessica Dawson, The Washington Post Tuesday, December 23, 2008; Page C01. And why should I care you ask? Because the Museum's painting of Philadelphia doctor John Hill Brinton by Thomas Eakins is being worked on in the same way. The painting of the Museum's first curator has been on loan to the Gallery since 1946, but recently has traveled to New York City and Italy for Eakins' shows.
I was down visiting the conservators and curator a few weeks ago when they uncovered some details in the painting that hadn't been obvious before. I told them I'd have no idea what they were and I didn't, but research goes on and I'll let them tell there story in their own time and place.