A metering device is disclosed for regulating the depth of flowable product coating a surface movable relative to the metering device by directing an air curtain towards the surface. The metering device comprises a body (10) having an interior chamber (12), an inlet for connecting the chamber to a source of gas under super-ambient pressure; and an opening (16) communicating with the chamber and having a mouth through which gas is discharged from the chamber towards the surface to form the air curtain. A porous or reticulated membrane (18) is secured to, or formed integrally with, the body of the device to lie in the path of the gas discharged through the mouth of the opening.