A model problem and an associated mathematical model relevant for the macrosegregation formation close to and at the surface in aluminium direct chill casting are presented. The model is used to study how the solute diffusion within and coarsening of the dendrites influence the macrosegregation formation due to exudation and solidification shrinkage. It is found that the Scheil limit leads to the thickest and most concentrated exudated layer. For alloys having finite (but slow) solute diffusion, the model predicts a macrosegregation close to that obtained in the lever rule limit. If the secondary dendrite arm spacing is assumed to be equal to that found after complete solidification, the thickness of the exudated layer and the associated solute depletion within the casting are severely overestimated. For cases in which there is no exudation, the positive segregation due to solidification shrinkage is more pronounced the less the local solid solute diffusion is. Coarsening is shown to be of less importance in quantifying the macrosegregation when there is no exudation.