Graffiti is not only ugly, it costs society millions of dollars to remove it. But graffiti on historic landmarks is worse because it often can't be removed using basic caustic solutions without damage to the underlying surface. Now a new, breathable coating could help preserve some of our most beautiful and priceless links to the past by providing them with an efficient, all-round protection against attacks by taggers.

While it might only take seconds to vandalize a wall or structure with spray on graffiti, it can take hours or weeks to remove it, let alone the cost of materials and man hours. Tags on porous natural stone or brickwork, as found in the majority of historic monuments, can be almost impossible to remove because the paint penetrates deep into the pores. Even pressure hoses or multi-component solvents can prove ineffective.

Learning from the past

The problem with special anti-graffiti polymer coatings, which have been on the market for several years, is that they create a hydrophobic seal that closes the pores, preventing the paint from adhering to the under-surface and allowing it to be wiped off. The downside of this treatment is that the building can no longer breathe as it was designed to, augmenting the risk of mold development or salt efflorescence.

“There are conflicting requirements for this kind of polymer coating – it mustn’t seal the pores, because it is important that there should be a continuous exchange of air between the building and the external environment, and at the same time it has to prevent the spray paint from penetrating the pores,” says Prof André Laschewsky, who heads the relevant research group at the Fraunhofer Institute for Applied Polymer Research IAP in Potsdam.

“The coating needs to be sufficiently resistant to withstand both weathering and mechanical cleaning. Moreover, since we’re dealing with historic landmarks, it must be possible to completely remove the coating from the walls if required, to restore them to their original condition with little effort and without damaging the structure.”

As part of an EU-sponsored project, Laschewsky’s team and partners from the Center of Polymer and Carbon Materials of the Polish Academy of Sciences in Gliwice and Zabrze have developed a polymer coating that meets these requirements. “Our innovative polymer film seals the pores in the substrate, so that graffiti paint doesn’t penetrate. But its micro-porous structure also creates a hydrophobic barrier that allows water vapor to escape from the building while at the same time preventing the infiltration of rainwater,” says Laschewsky.

In the trials, coated samples of ancient stone and brick were repeatedly covered with graffiti – which was removed completely each time. The coating was then removed from the surface using a diluted brine solution that modified its chemical composition and allowed it to be washed off.

Maybe a penalty for those caught tagging these buildings should be having to fully cover the vandalized building, and others like it, with the polymer coating - using a very small paint brush.