Melt flow instabilities associated with surface defects for polymer extrudates and commonly referred to as the "Sharkskin Effect," are modeled. The relaxation oscillation model of Molenaar and Koopmans, developed to describe "Spurt" defects is expanded to provide the theoretical framework for the description of surface defects. It is shown that a nonlinear viscoelastic constitutive equation may give rise to a boundary layer, which is unstable. The presence of an oscillating boundary layer is one of the many possible solutions of the momentum equation indicating the need for additional selection criteria to define which one will be selected by the polymer system. The energy balance equation is suggested to provide the extra criterion. The current macroscopic approach may form the basis for the reconciliation of the debate on the origin of extrudate surface melt now instabilities as either a "slip-at-the-wall" or a non-monotone "constitutive equation" phenomenon.