smooth (something) onto (someone or something)
To spread some substance evenly across the surface of something or on someone's skin with a rubbing motion. I smoothed the peanut butter onto the crackers and sprinkled raisins all over them. You'll need to smooth this ointment onto the burned skin three to four times per day.
smooth something onto someone or something
and smooth something onto spread or flatten something onto someone or something. Ted smoothed the suntan lotion onto Alice, who lay on a towel in the sand. He smoothed on some lotion.