to coat a surface by spreading something over it: to plaster, daub, or overlay, most often mortar on a wall
The picture behind this verb is a workman with a handful of wet mortar, spreading it across a wall until the surface is sealed. Leviticus uses it in the plainest possible setting. A house whose walls have broken out in a spreading blight is stripped back to bare stone, new stones are set in, fresh dirt is mixed, and then the owner plasters the house. The priest comes back afterward to see whether the trouble returns.
Because the word names the act and not the substance, it stretches to whatever is being spread. David sets aside gold from Ophir and refined silver to coat the walls of the buildings he is preparing for the temple. Isaiah uses it of eyes: people who bow to a block of wood cannot see, because something has been smeared across their eyes and their minds have been closed. Two neighboring words stay in their own lanes. צָפָה is the usual word for fastening a skin of gold or bronze over wood, and מָשַׁח is the word for rubbing oil on a person to set him apart, while this verb is the one for spreading a coating that clings to a surface.
Ezekiel gives the word its most memorable use. Judah's prophets are pictured as builders who run up a flimsy partition and then coat it over, so that from the street it looks like a wall that will stand. God promises a driving rain, hailstones, and a gale. The coat washes off, the wall goes down, and the plasterers go down inside it. The charge is not that they coated a wall, since coating a wall is honest work in Leviticus. The charge is that there was nothing behind the coat: they told the people peace when there was no peace.