A method of making a gas separation system is described. The method comprises the steps of (1) providing a porous support having an initial mean pore size and initial surface roughness and (2) applying a particulate material to a surface of the porous support to (a) functionally reduce the mean pore size of the support and (b) functionally reduce the measurable surface roughness of the support. Additional layers of particulate material of decreasing size are applied to further reduce the mean pore size and reduce the roughness of the support. When the support reaches the desired level of smoothness a thin membrane of gas selective material is deposited thereon. The membrane and support are then annealed under conditions that prevent or substantially reduce cracking of the membrane during commercial use.