The progressive transition from Excimer to EUV lithography is driving a need for flatter and smoother photomasks. It is proving difficult to meet this next generation specification with the conventional chemical mechanical polishing technology commonly used for finishing photomasks. This paper reports on the application of sub-aperture CNC precessed bonnet polishing technology to the corrective finishing of photomask substrates for EUV lithography. Fullfactorial analysis was used to identify process parameters capable of delivering 0.5 nm rms surface roughness whilst achieving removal rates above 0.1 mm3/min. Experimental results show that masks pre-polished to 300~600 nm P-V flatness by CMP can then be improved down to 50~100 nm P-V flatness using the automated technology described in this paper. A series of edge polishing experiments also hints at the possibility of increasing the quality area beyond the 5 mm defined in the official EUV photomask specification.