When two structural components are to be joined, for example, when an aircraft skin section is to be secured to a stringer by an adhesive bonding, it is important that the joining surface areas are plane to avoid the formation of air inclusions in the joint because air inclusions weaken the mechanical strength of the joint. Thus, a surface topography that is not plane must be smoothed out prior to the joining. For this purpose the surface areas to be joined are optically scanned to provide topographical information for each surface area that needs to be smoothed out for providing a plane joining surface. The topographical information is processed to ascertain the configuration and volume of the surface topography that needs to be filled with a filler material to produce a plane surface or at least a smooth surface without hills and dales. A respective control signal is produced and fed to a dosing equipment which supplies the exact volume of filler material required for planing or smoothing the surface topography. When smoothing is completed the two components are ready for joining for example by the application of pressure and heat to the filler material.