Abstract: A spreader tool is provided for spreading viscous material on the surface (5?), for example an edge (5?), of a wind turbine blade (5). The spreader tool (52) comprises at least one bendable spreader wing (54A, 54B), for example provided as a fin-ray construction, for pressing a flexible band (53) against a curved section of the blade (5) and for dragging viscous material along it by the flexible band during movement of the spreader tool (52) along the blade (5) surface (5?). The spreader tool is advantageously part of a robot system used to work the surface (5?) of the blade (5).