A tool for applying mastic to drywall, having a flexible trowelling bar for smoothing and shaping the applied mastic. The curvature of the trowelling bar is controlled by the rotation of a cam that is positioned on a rigid support and that is directly coupled to the trowelling bar through a link. Springs are positioned between the rigid support and the trowelling bar to maintain tension on the link during rotation of the cam. The springs exert pressure on the trowelling bar through plungers having rounded feet that maintain consistent contact with the trowelling bar as the curvature of the trowelling bar changes.