The spatial resolution aspects of the local modification of porous materials by electron induced graft-polymerization were studied by a combination of experiments and numerical simulations. Using blocking masks, only selected regions of the material were exposed to radiation and subsequently grafted. The main focus of this study is the application to gas diffusion layers, a carbonaceous ~200 µm thick porous substrate widely used in fuel cells, with the goal of improving water management by locally tuning the wettability. The comparison of experiments performed with different electron energies and corresponding simulations shows good agreement, identifying the energy threshold necessary to graft through the material to be approximately 150 keV. The impact of electron energy on spatial resolution was studied, showing that the blurring effects due to electron scattering reach a maximum at around 200 keV and are reduced at higher electron energies. Finally, the numerical simulations were used to define the conditions necessary to selectively graft only parts of bi-layer fuel cell materials.