The invention relates to a method of preparing a surface to be treated such as a surface to be painted. The surface is initially divided into so-called patches by a method known. Subsequently, the patches are grouped in so-called main faces by the method according to the invention. The main faces are used for defining a number of so-called virtual surfaces, also by the method according to the invention, and the virtual surfaces are used to establish lines of treatment along the surface, such as paint lines, said lines of treatment defining the subsequent motion of the tool for treatment.