A guide device is adapted to be removably attached to a flat box device for use in applying mastic adjacent an outside corner or edge. The guide device includes a generally S-shaped guide bar, a bracket, and a spring. The bracket is adapted to attach to the flat box device. The guide bar is pivotably and slidably retained by the a first bracket portion. The spring is adapted to bias a first guide bar end of the guide bar away from the first bracket portion of the bracket. The guide bar extends across a width of the flat box device and the first guide bar end extends below the flat box device when the guide device is operably installed on the flat box device. The spring biases the guide bar in a lateral direction corresponding to the width of the flat box device when the guide device is operably installed on the flat box device.