The Smithsonian Institution's monumental 1796 Lansdowne portrait of George Washington is scheduled to undergo an 18-month restoration including X-raying and the removal of a discoloring layer of varnish, the National Portrait Gallery said on Monday.
Restoration of the portrait of the first US president by Gilbert Stuart will start in February 2016 and funding will be provided by Bank of America Corp, the museum, which is part of the Smithsonian Institution, said in a statement.
The oil painting, the centerpiece of the National Portrait Gallery's "America's Presidents" permanent exhibition, measures 8 feet by 5 feet (2.44 meters by 1.52 meters) and was done during Washington's second term in the White House.
It shows him in a black suit, symbolizing that his role was not that of a king but of an elected official, the museum said.
Senator William Bingham of Pennsylvania and his wife bought the painting and sent it to the Marquis of Lansdowne, an English supporter of US independence, in the year it was created.
Several layers of varnish remain on the painting's surface and Washington's black coat has become discolored and uneven, the museum said. It will be visible at times during the restoration in a conservation centre's laboratory.
The painting was restored before its long-term loan to the National Portrait Gallery in 1968. The museum acquired it in 2001 and it was installed in 2006. More than a million visitors view the portrait each year.