A systematic variation of preparation conditions for epitaxial Zn0.9Co0.1O films grown by reactive magnetron sputtering on c-plane sapphire has been carried out to study the correlation of crystalline perfection with the corresponding magnetic properties. The crystalline perfection of the resulting films was found to vary over a wide range, nonetheless all samples were found to be paramagnetic. To further extend the study, three samples, which were paramagnetic in the as-grown state, were subsequently implanted using Cu, Li, and Zn ions. Only Zn ion-implantation was found to slightly alter the magnetic properties at low temperatures, while synchrotron-based techniques could not evidence the formation of a secondary, metallic Co phase. The origin of this weak, low-temperature magnetism is more likely to be carrier-mediated rather than defect-induced.