The tools Mr. Moser keeps in his Brooklyn studio reveal the complexity of his trade: a ladder; a thick roll of cotton; jars of emollients, solvents and glues; an exhaust to rid the room of noxious fumes; a lamp whose hue mimics daylight; and dried pigment, palettes and brushes of all shapes and sizes.

Image Kenneth S. Moser restores the historical portraits that adorn the hallways, ceremonial rooms and workspaces at City Hall. Credit... Ozier Muhammad/The New York Times

He is part chemist, part repairman and part artist.

“There’s a little bit of connecting the dots in this work,” Mr. Moser said. “It’s not an exact science that you say, one of this, two of that, three of these and you’re done.”

Unlike plastic surgeons, who strive to turn back time, conservators aim to lessen time’s damage without altering the nature of a piece of art. Theirs is an unhurried, measured and solitary task: They toil alone, accompanied sometimes by music (for Mr. Moser on a recent Sunday, it was Bach’s Cello Suite No. 1 in G Major) and always by the faces and sceneries they caress with gentle brushstrokes.

The urge, Mr. Moser said, is to keep going, to keep on mending a painting’s imperfections even when no one else can notice them. It is an internal battle between obsession and restraint that he knows, for the sake of fidelity to a portrait’s epoch and a painter’s style, can end only one way.

“Once I start to get a palette”  the white on a soldier’s uniform is no longer yellowed or a gushing fountain in the background is unveiled from under layers of varnish and dust  “then I know it’s time to stop,” Mr. Moser said.