The paper deals with the topic of a Gas Insulated Substation (GIS) optimized for HVDC. The electric field distribution of DC systems is mainly controlled by the conductivity k of the dielectric media. Surface charge accumulation leads to field distortion. In order to prevent this phenomenon on the spacer of such systems, a conducting coating has been realised. Investigations are made with a real 145 kV-GIS-setup. The influence of the surface coating on the field distribution can be proved with this experimental setup. Numerical calculations based on the Boundary-Element-Method (BEM) are made to investigate the influence of surface resistivity and surface charge on the field distribution of a spacer. The charge accumulation on an uncoated spacer is measured, simulated and discussed. With a coated spacer it is possible to prevent this charging.