GP Plasma needed to improve the cross-web uniformity of some planar cathodes. The as-built uniformity on a web roughly the length of the straightaways of the target was in the range of +/-12% due to the deposition rate drop-off at the turnarounds. With the planar cathodes and surrounding deposition area modeled in COMSOL Multiphysics, static magnetic fields and electric fields were used to determine the Lorentz forces. The magnitude of these forces on the surface of the target was then used to generate a molecular flow model to simulate material deposition from the cathodes onto the substrate. While this was a good first-order uniformity plot that followed the trend of the measured uniformity on the web, it was more uniform than actual results due to the lack of a cross-corner effect. Nevertheless, trim shields designed to improve the uniformity by a factor of two in the model translated directly to the measured results.