Removing Graffiti from Historic Masonry (Part F)
Anti-graffiti or barrier coatings are intended to facilitate the removal of graffiti from porous as well as non-porous surfaces. These coatings are most commonly transparent, but may also be pigmented. They are available in a variety of formulations designed to serve different needs. The useof barrier coatings to protect graffiti-prone historic masonry surfacesmay seem to be an easy preventive solution to a persistent graffiti problem. However, for the most part, these coatings are not the panacea that some advertising might suggest. Some of them simply do not work, and others may cause physical or aesthetic changes or damage to the masonry.
Transparent Coatings. Transparent coatings serve as a barrier between the masonry surface and graffiti, preventing graffiti from penetrating into the masonry. They are also intended to make graffiti removal easier since most graffiti does not adhere well to them. Generally, graffiti applied over transparent barrier coatings can be removed with low-pressure water and a detergent, or with a solvent.
There are basically two kinds of transparent barrier coatings: temporary and permanent. Temporary, or "sacrificial" coatings are removed when graffiti is removed and then must be reapplied. Permanent transparent barrier coatings are more resistant to the water or solvents used to remove graffiti, and remain on the masonry surface when graffiti is removed (althoughthis type of coating also must usually be reapplied after several cleanings).A third type of transparent barrier coating combines temporary and permanent coatings, based on a two-part system. A water-based acrylic sealer is firstapplied to the masonry surface, after which a sacrificial layer consistingof a polyethylene wax emulsion or dispersion coat is applied over the sealer. When graffiti is removed, the sealer coat remains on the masonry, but the sacrificial coat dissolves and is removed with the graffiti, and thus must be reapplied.(With this two-part system, even the first coat will eventually wear off after multiple cleanings, and must also be reapplied.)
Unfortunately, in application, there are a number of negative aspects of transparent barrier coatings that generally prevent their being recommended for use on historic masonry. First, clear coatings may alter the color of the masonry surface and add agloss that may be highly visible, or apparent only in certain lighting conditions or when it rains. Second, clear coatings may reduce the water-vapor permeability of the masonry, thereby contributing to possible water-related deterioration. Third, the coating may discolor and change over time. Exposure to ultra-violet light can cause a coating to yellow; dirt build-up may darken the treated surface; and some coatings acquire a sheen when rubbed or brushed against. Such changes are especially noticeable when only aportion of the building has been coated. Furthermore, if coatings are not maintained on a regular basis, usually through periodic removal and reapplication, many coatings tend to fail. What often results is an uneven, "patchy"look to the masonry that can have a very negative impact on the character of the historic building.
Despite these potential drawbacks, there may be some instances in whichthe graffiti problem or frequency of occurrence is so severe that application of a transparent barrier coating on historic masonry may be worth considering.Some water-base polysaccharide coatings, and silicone and silicone-base coatings have been used with success on masonry structures. They are essentially invisible, and do not change the natural appearance of the masonry. Although less durable than solvent-borne coatings, they are water-vapor permeable(breathable), and may be reapplied to the masonry surface immediately after removing graffiti, while the surface is still damp.
However, extreme caution must be exercised before applying a transparen tbarrier coating. Experimental test applications should always be tried first on discrete areas that are not highly visible, and the treated areas evaluated over a period of time. Laboratory test results on the performance of coatings applied to samples of like masonry types may be useful to some extent. But because the tests are carried out in a controlled environment, they may not be as accurate or reliable as tests actually carried out on-site where the factors of weather and pollution are the same as those at the location where the coating will be used. If circumstances warrant, andthe use of a barrier coating is determined necessary, an architecturalconservator should evaluate the test performance of a variety of coatingsbefore selecting one to be applied to historic masonry. Because of the potential for disfigurement, owners of landmark-designated buildings are required by some preservation review boards and landmark commissions to obtain approval before they apply a barrier coating.
Pigmented Coatings. A pigmented barrier coating may be used onmasonry as a permanent, preventive barrier coating, or as a temporarymeans of concealing graffiti until it can be removed.
Like a transparent barrier coating, a pigmented barrier coating facilitates the removal of graffiti because graffiti does not adhere well to it. Pigmented barrier coatings that are water-vapor permeable may sometimes be used asa permanent barrier coating on non-historic masonry where there is frequent recurrence of graffiti, and when constant surveillance is not possible (Fig. 10). Although there are some instances in which pigmented barrier coatings may be appropriate on painted historic masonry, they are not recommended for unpainted historic masonry because they will change the appearance of the masonry. There is also another kind of pigmented coating that is specially formulated to be used as a temporary measure to conceal graffiti that cannot be removed right away. This temporary, vapor-permeable paint is removed when the graffiti is removed.
Pigmented coatings are also not generally recommended as a permanent measure to cover up graffiti. Some graffiti materials, particularly feltmarkers, bleed through the coating; and repeated applications of the coatingor paint can result in a heavy paint build-up on a masonry surface. Another disadvantage of using paint or a pigmented coating to hide graffiti isthat it usually appears as an obvious patch on unpainted masonry and tends to attract more graffiti unless the paint can be applied in a discrete, and well-defined area (Fig. 11). If incompatible with either the masonry or the graffiti, such a coating may peel off the masonry surface in an unsightly manner. Like transparent coatings, pigmented coatings may bedifficult or impossible to remove completely once their performance or appearance is no longer satisfactory.