An investigation of plasma-polymers of hexamethyldisilane and oxygen was conducted, to allow optimization of the interfaces and surfaces of these plasma-polymers. For this study galvanized steel was used as a substrate. During this work the chemical structure (X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy), morphology (atomic force microscopy) and water contact angles of films deposited at different pressures and oxygen partial pressures were measured. These studies have shown that gas pressure and oxygen partial pressure can be used independently to determine the surface roughness of plasma polymer films derived from hexamethyldisilane (HMDS). At high pressures and low oxygen partial pressures roughness of the films increased. The roughness is then also shown to determine the water contact angle within a certain range of oxygen concentrations although the chemical structure of the films changed. With a high roughness a contact angle of 160° was observed. The roughness was determined only by the plasma-polymer layer. Despite this high contact angle the samples could be painted; the adhesion of rough films onto the substrate was, however, reduced.