A fluorinated polyurethane, based on perfluoropolyether (PFPE) blocks and containing carboxylate side groups, reacts with polyfunctional aziridines affording a crosslinked material. The reaction takes place in water microdispersion at room temperature and allows the PFPE derivative to be modified directly on the stone surface. The application of this material on the surface of different stones improves not only the substrate’s hydrophobicity (as expected for a PFPE derivative) but also the resistance to the soiling, which could be caused by “graffiti” drawings. The treatment generates a protecting layer which resists on the stone surface even after several repeated staining/cleaning cycles, giving rise to a durable “anti-graffiti” effect. The curing process improves the effectiveness of the fluorinated polymer: in the absence of the aziridine curing agent the considered fluorinated polymer does not exhibit a satisfactory anti-graffiti activity.