During casting of alurnimum extrusion ingots the surface against the mould experiences a pull-in force that magnifies the air-gap during solidification close to the mould surface. This is a global phenomenon that results in early and large air-gap formations compared to the shape-casting situation. Due to the semi-solid surface created under such conditions, exudation through the surface may appear. In this study a coupled heat and fluid flow, stresses and deformation modelling tool are applied on the process. Results from the mechanical calculation are back-coupled to the thermal boundary conditions. The metallostatic head is the driving force for exudation through the dendritic network and the resulting fluid flow through this network is used to calculate a dynamic thickness of the exuded layer. Measurements from two different alloys, with rather small changes in composition, but with large variations on surface quality, are compared with the modelling results.