[When the galleries closed] Then a patchwork of drop cloths went down, and a forest of scaffolding went up. Doors were shifted a few feet to the left or the right, improving the sight lines in some of the galleries. About 1,400 skylights on the roof were replaced, many for the first time in 84 years. More than 7,000 glass panels in the ceilings of the 45 galleries were also replaced. Contractors did masonry repairs and repointing on interior and exterior walls. A new heating and air-conditioning system was installed, with more than 5,000 linear feet of ductwork.

All that took five years with a budget of $150 million, and then the art had to be moved back. Redoing the walls took 900 gallons of paint — two shades of blue, along with red, purple and gray for the trim. Rehanging the paintings took 10,000 feet of wire.