To spread some substance evenly across the surface of something with a rubbing motion. A noun or pronoun can be used between "smooth" and "on." I smoothed on the peanut butter and then sprinkled raisins on top. You'll need to smooth this ointment on the burned area of the skin three to four times per day.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.