Abstract: A mastic applicator or "flat box" for use in application of mastic to tape drywall joints includes a container for storing and dispensing mastic. The container has a pressure plate which both supports the apparatus from a handle and applies pressure to force the mastic from an opening that extends across the direction of motion of the tool. To distribute and contour the mastic a flexible blade assembly extends along the rearmost edge of the opening and is positively driven through a range of infinitely adjustable positions by a means of a lever and camming linkage arrangement. The entire adjustment mechanism is itself adjustably mounted so as to selectively adjust the entire adjustment range.