A new coating process involving a short stretching distance (1 mm), a high draw ratio (around 200) and a pressure differential between the two sides of the polymer film is considered. The resulting thin molten polymer film (around 10 micrometers) is set down on a solid primary film and then covered by another solid secondary film. In experimental studies, some defects involving either periodical fluctuation in the thickness of the coated layer or even localized web rupture may be observed. The processing conditions influence markedly the onset and the development of these defects and modeling will help understanding their origins. The membrane approach which has been commonly used for cast film modeling is no more valid and two dimensional time dependent models (within the thickness) are developed in the whole domain (upstream die and stretching path). A boundary-value problem with a free surface for the Stokes equations is considered and stability of the free surface is assessed using two different numerical strategies : a tracking strategy combined with linear stability analysis involving computation of leading eigenvalues, and a Level Set capturing strategy coupled with transient stability analysis. These models allow understanding defects observed for some processing conditions.