The CIRS instrument to be flown on the Cassini mission to Saturn is a cryogenic spectrometer with far-IR (FIR) and mid-IR (MIR) channels. The CIRS FIR channel is a polarizing interferometer that contains three polarizing grid components. These components are an input polarizer, a polarizing beamsplitter, and an output polarizer/analyzer. THey consist of a 1.5 micron thick substrate with 2 micrometers wide copper wires, with 2 micrometers spacing, photolithographically deposited on the substrate. Mylar and polypropylene were chosen as the flight candidate substrates. After the testing was performed, mylar was chosen over polypropylene for the CIRS instrument due to a better cryogenic reflectance performance. These elements were fabricated at Queen Mary and Westfield College in London. This paper details the flight qualification of the mylar substrate and the characterization of the polypropylene substrate. Performance tests included cryogenic optical flatness, cryogenic polarization sensitive reflectance and transmittance measurements. Environmental tests included vibration, acoustic, humidity, and radiation survivability.