A method comprising: (a) depositing a film of a gas-selective material comprising a gas-selective metal upon a surface of a tubular porous support, thereby providing the tubular porous support with a gas-selective base membrane layer; (b) annealing the gas-selective base membrane layer, thereby forming a first annealed gas-selective membrane layer; (c) forming a first abraded membrane surface by polishing the first annealed gas-selective membrane layer under a first controlled polishing condition with an abrading medium comprising a rotary fibrous buff that includes a plurality of abrasive particles having a size in the range of from 150 mesh to 280 mesh that adhered to a fibrous support with a polymeric binder; and wherein the first controlled polishing condition comprises regulation by use of a robotic polisher of the tubular porous support, the tubular porous support being rotated about its axis at a speed within the range of 100 rotations per minute (rpm) to 1000 rpm, while pressing the rotary fibrous buff against the tubular porous support at a contact angle, being the angle that the rotational axis of tubular porous support makes relative to the ground or other point of reference when it is in contact with the rotary fibrous buff, in the range of from 0° to 90° and at a force within the range of from 2 to 9 kg (5 to 20 pounds) when rotating the rotary fibrous buff at a rotation speed within the range of 50 to 2500 rpm and moving the rotary fibrous buff or the tubular support relative to each other at a lateral speed at their point of contact in the range of from 1 millimeter per second (mmps) to 250 mmps for from 1 to 20 repetitions, a repetition being where the rotary fibrous buff remains in contact with the tubular porous support and passes along one full length of the tubular porous support from one end to the other end and back again; (d) depositing a film of the gas-selective material upon the first abraded membrane surface, thereby forming a first overlaid membrane layer; (e) annealing the first overlaid membrane layer; and (f) successively iterating operations (c) and (d) until a leak-tight membrane system is formed, a successive abraded membrane surface and a successive overlaid membrane layer being formed at each iteration, each successive overlaid membrane layer being annealed before performing a subsequent iteration. (a) depositing a film of a gas-selective material comprising a gas-selective metal upon a surface of a tubular porous support, thereby providing the tubular porous support with a gas-selective base membrane layer; (b) annealing the gas-selective base membrane layer, thereby forming a first annealed gas-selective membrane layer; (c) forming a first abraded membrane surface by polishing the first annealed gas-selective membrane layer under a first controlled polishing condition with an abrading medium comprising a rotary fibrous buff that includes a plurality of abrasive particles having a size in the range of from 150 mesh to 280 mesh that adhered to a fibrous support with a polymeric binder; and wherein the first controlled polishing condition comprises regulation by use of a robotic polisher of the tubular porous support, the tubular porous support being rotated about its axis at a speed within the range of 100 rotations per minute (rpm) to 1000 rpm, while pressing the rotary fibrous buff against the tubular porous support at a contact angle, being the angle that the rotational axis of tubular porous support makes relative to the ground or other point of reference when it is in contact with the rotary fibrous buff, in the range of from 0° to 90° and at a force within the range of from 2 to 9 kg (5 to 20 pounds) when rotating the rotary fibrous buff at a rotation speed within the range of 50 to 2500 rpm and moving the rotary fibrous buff or the tubular support relative to each other at a lateral speed at their point of contact in the range of from 1 millimeter per second (mmps) to 250 mmps for from 1 to 20 repetitions, a repetition being where the rotary fibrous buff remains in contact with the tubular porous support and passes along one full length of the tubular porous support from one end to the other end and back again; (d) depositing a film of the gas-selective material upon the first abraded membrane surface, thereby forming a first overlaid membrane layer; (e) annealing the first overlaid membrane layer; and (f) successively iterating operations (c) and (d) until a leak-tight membrane system is formed, a successive abraded membrane surface and a successive overlaid membrane layer being formed at each iteration, each successive overlaid membrane layer being annealed before performing a subsequent iteration.