dab at (something)
To repeatedly touch something lightly and carefully, as if gently applying or removing something. After lunch, I tried to clean my son up by dabbing at his jam-covered face. I only dabbed at the mirror so that I didn't make the smear worse.
dab at something
to touch or pat something. The painter dabbed at the canvas, making little changes here and there. Don't just dab at the wall, spread the paint on!