Example of low-order coating variation on the TMT primary mirror. We generate a random set of Zernike coefficients from Z1–Z4 and apply it to each segment using the prysm numerical optics package. The error is normalized to have a peak-to-valley of 10% of the nominal thickness and then added to the nominal thickness. Quantifying the impact of these errors on coronagraphy and polarimetry is important to understanding the as-built performance of the GSMTs.