Abstract: Adapter for a mastic applicator and finishing tool flat box having a guide plate and corner guide assembly adapted to be mounted on opposite side and end walls of the flat box. Each corner guide is slidable between an extended position in which it has an end portion that can be projected beyond the end wall on which the assembly is mounted. The extended end portion is adapted to straddle a corner bead to guide the movement of the flat box along the corner bead during mastic filling of the corner bead. The corner guide end portion position in retracted position being out of contact with the wall board thereby permitting normal flat box mastic dispensing movement across the flat wall board surface. Each adapter guide plate having a flange portion overlying the flat box wall end on which it is mounted, the surface of the flange functioning as wear shoes adapted to ride on the abrasive wall board surfaces during normal flat box operation and during bead filling operation.