Municipal authorities, public companies and private homeowners pay millions of dollars each year to remove graffiti from buildings and other structures.

Municipal authorities, public companies such as railway and postal services, housing complexes, and private homeowners pay millions of dollars each year to remove graffiti from buildings and other structures.

Graffiti removal is largely influenced by the substrate, the paint used and the cleaning method. Key factors are the surface roughness, porosity and absorbency of the substrate on which graffiti has been applied.

The type of paint used also has a considerable influence on the success of the cleaning process. Generally, spray paint can be removed relatively easily from smooth, nonabsorbent substrates using suitable cleaning agents. However, marker pen ink presents more of a problem because of its composition.

There are mechanical and chemical cleaning methods for removing graffiti from walls and buildings. The chemical methods involve the use of organic solvents or cleaning pastes to dissolve and remove the paint. It should be kept in mind that if the wrong cleaning agent is used, it may dissolve the colors and soak them deep into the substrate.

The mechanical processes involve the use of abrasive materials. The amount of substrate removed depends on the fineness of the abrasive and the application pressure used. Wet and dry blasting processes are used, both of which have inherent disadvantages in terms of cost and health risks respectively. Dry blasting results in the formation of large amounts of dust, which can only be reduced by means of a suitable extraction system. In the case of wet blasting, the water must be captured, treated and disposed of in a suitable manner.

Cleaned sections are usually lighter in color than the surrounding area. It is often impossible to remove graffiti completely from porous substrates because it has penetrated too deeply (see Photo 1).

Prevention is better than cure, goes the old saying. By analogy, it is better to protect surfaces before the graffiti artists actually strike. Two approaches are available in the form of temporary and permanent protective systems.

The temporary systems function according to the sacrificial principle. Protection is provided by a coating — often wax-based — that does not crosslink chemically and is applied on the painted wall. If cleaning becomes necessary, the coating is removed together with the graffiti.

These temporary systems have the benefit of having little impact on the appearance of the wall. As they are applied in thin films, they also have little effect on the water vapor diffusion properties. This means that any water that may penetrate the wall can escape easily. As a rule, walls protected by a temporary system are cleaned with high-pressure washers. The water must be captured and disposed of properly. The cleaning operation should therefore be carried out by a specialist company.

The disadvantage of this approach is that the coating must be renewed after cleaning. Furthermore, temporary systems do not seal the substrate. It is therefore possible that the paint may penetrate deep into the pores of the substrate and cannot be removed completely.

Most of the permanent systems are lightfast and weather-stable coatings based on polyurethane. In contrast to the temporary systems, they provide long-term protection. In other words, the coating must not be renewed each time the wall is cleaned.

The durability of two-component polyurethane coatings results from the fact that they are reactive resin systems. After application on the wall, the two components in the system react to form a crosslinked film with a smooth nonporous surface. This results in outstanding chemical resistance. The spray paints favored by graffiti artists cannot penetrate the film. Even the dyes used in many marker pens, which are far more aggressive than spray paint, can be removed easily.

Special cleaning agents are used to remove the graffiti from the smooth PUR-coated walls. The cleaning agent must be tailored to the coating.

These permanent systems are high-quality coatings that seal the pores of the substrate. Transparent formulations give walls a wet look that is not always acceptable. Special primers help minimize this effect but cannot eliminate it entirely.

The polyurethane coatings reduce the water vapor diffusion properties of the treated walls compared with untreated walls. This may result in damage caused by moisture if the moisture barriers prescribed by building regulations are no longer intact. The diffusion value of the protective system depends on the raw materials and formulation used. If correctly selected, the water vapor permeability of the coating is high enough that it exceeds critical values.

Solventborne cleaning agents are needed to remove graffiti. The cleaning process itself is simple: the graffiti is wiped off the wall using a cloth soaked with the cleaning agent. Both gloss and color are unaffected. The waste from the cleaning process must be disposed of in the same way as leftover paint. The combined use of cleaning agents and high-pressure washers with integrated water capturing devices means that even large areas can be cleaned without any problems.

The polyurethane systems provide more than just good protection against graffiti. They also have a very good CO₂ barrier effect, effectively protecting concrete structures against carbonation damage. They also prevent corrosive substances such as acid rain from penetrating the substrate. Pigmented formulations fulfill additional decorative functions. Good weather stability is a standard feature of durable protection systems.

In contrast to normal masonry paint, the permanent polyurethane systems are not thermoplastic. This means that they do not soften when exposed to heat, such as from sunlight. As a result, any dirt on the surface cannot bond with the coating and is washed away by rainfall. Compared with standard emulsion paint, these coatings are therefore less prone to soiling.

Bayer AG has invested successfully in the research and development of raw materials for the formulation of polyurethane coating systems. The company’s work has resulted in Desmodur® and Desmophen® for solventborne systems, and Bayhydur® and Bayhydrol® for aqueous systems. These products are used by paint manufacturers in their formulation of easy-to-clean coatings, as they are known at Bayer because of their properties.

Easy-to-clean coatings based on these polyurethane raw materials satisfy the highest requirements. They have proved their value in practical application, as shown by the example of the parking structure at the Cologne Zoo (see Photo 2). In 1992, an artist created a fantasy zoo on the walls there. The easy-to-clean paintings are an attractive sight for arriving visitors.

The pleasant yellow-green coating that protects the concrete walls of the parking structure serves as the background for rhinos, snakes and other animals. Users of the structure like the friendly light-colored atmosphere compared with the gloomy dark gray normally found in parking structures. Their sense of safety and well-being is enhanced.

Despite this atmosphere and the fact that there are no large areas in a single color, the parking structure has been targeted by graffiti artists on many occasions. However, the permanent polyurethane coating that protects the concrete makes cleaning the walls an easy task. A cleaning agent tailored to the coating is wiped over the wall with a cloth to reinstate the paintings shown in Photo 3. Even after seven years of such “attacks,” the wall coating looks as good as new.

For more information on antigraffiti systems, contact Bayer Corp., Carl Angloff, 100 Bayer Road, Pittsburgh, PA 15205; fax 412/727.2838; or Harald Niesel, Bayer AG, Coatings and Colourants, D-51368 Leverkusen, Germany; fax 49/214.30.5.27.78.