WallPaint 2 is the second iteration of a robot Billy and I started at MakeHarvard. We wanted to create a robot that could paint walls in your house, for no other reason except that we felt it could be done.
WallPaint has a paintbrush mounted on a leadscrew which can be raised and lowered by the Arduino. The leadscrew assembly rides on top of the frame on a wheeled cart that is powered with a timing belt. A peristaltic pump ensures the paintbrush has paint on it at all times.