Fabrication of nearly defect-free mask blanks is one of the most significant challenges facing the commercialization of extreme ultraviolet lithography (EUVL). Despite significant advances in our ability to clean substrates, the incoming substrate contributes more defects than the multilayer to the total number of defects on our lowest defect density mask blanks. This is because cleaning processes are ineffective against substrate pits, which dominate the substrate defect distribution. Fortunately, defect mitigation methods have been developed that use a coat-and-etch process to smooth substrate pit and particle defects. We have designed and installed a process module specifically for smoothing substrate pits and particles. This process module has several new features, such as the ability to isolate the etch source during the deposition steps, and should enable cleaner planarizations than those done before. Currently, the greatest challenge for us is to demonstrate that the smoothing process can be rendered clean enough for manufacturing. We will present results on the particles added during planarization and the composition of these particles, which is critical to identifying their origin and eliminating them.