Two main types of morphological features resulting in surface roughness were observed on alumina discs used as supports for zeolite membranes. These features can be described as hills and pits and it was shown that defects as cracks formed in the zeolite film at these locations of the support. It was demonstrated that the roughness of the support can be reduced significantly by a polishing strategy developed in this paper. Finally, zeolite MFI membranes grown on the polished support shows remarkably improved quality as compared to films grown on non-polished supports.