Though a large variety of methods has been developed to model 3D objects, most of them are based on the construction of solid objects or curved surfaces. This work is part of a sketch-based project for modeling arbitrarily polygonal shapes. The method for modeling 3D objects is based on the user drawing or removing of edges. This paper addresses the automatic computation of the 3D polygonal surface model from the 3D wire frame. A straightforward incremental algorithm is proposed allowing an update of the surface model for each modification of the 3D wire frame. Objects of any genus, or topological type are handled by the method.