More and more research works on assembly tolerance simulation is considering not only orientation and position deviations, but also form errors. In this paper, we describe a process to generate skin model shapes, i.e. 3D models with form error, from the CAD model of a part. We study several different methods used for generating discrete models with defects, and these methods are applied to an identical CAD model to point out their differences. The work will be of benefit for choosing and applying of these different generation approaches, and for automating the generation of skin model shapes.