Anti-graffiti coatings on stones for historical buildings in Turin (NW Italy)

The application of anti-graffiti products to stones belonging to architectural heritage is a common procedure that is currently complementary to traditional graffiti removal treatments, such as chemical and mechanical cleaning. In this study, two anti-graffiti coatings (a sacrificial product and a permanent one) were tested on four stones (with a different texture, mineralogy, and surface finish) commonly found in the historical city center of Turin (Italy). In order to evaluate the effectiveness of the anti-graffiti products, the removal of two graffiti paints with different compositions was tested. The results of the cleaning procedures performed on the surfaces coated with anti-graffiti products were evaluated, considering both the graffiti remains and the alterations induced on the surface. Chemical cleaning based on the use of a low-toxic ternary solvent mixture was applied on the unprotected stones for a comparison with the results obtained on the surfaces coated with anti-graffiti products. The samples were observed under stereomicroscopy and ultraviolet fluorescence photography and all of the treated surfaces were evaluated by roughness measurements, the contact sponge method, static contact angle measurements, and scanning electron microscopy. The composition of the anti-graffiti product, the graffiti paint to be cleaned, and the remover recommended by the manufacturer proved to be key factors for the cleaning effectiveness achieved on coated surfaces. Moreover, to a lesser extent, the mineralogy, texture, and surface finish of the stone also influenced the results of the cleaning procedures. The sacrificial anti-graffiti product enhanced the cleaning effectiveness on all stones if compared to uncoated surfaces; however, the permanence of coating remains on the surface after cleaning proved to be critical. Regarding the use of the permanent anti-graffiti products, intense disparate results were achieved, depending on the graffiti paint composition.