Removing Graffiti from Historic Masonry (Part F)
Barrier Coatings
Anti-graffiti or barrier coatings are intended to facilitate the removalof graffiti from porous as well as non-porous surfaces. These coatingsare most commonly transparent, but may also be pigmented. They are availablein 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 someadvertising might suggest. Some of them simply do not work, and othersmay cause physical or aesthetic changes or damage to the masonry.
Transparent Coatings. Transparent coatings serve as a barrierbetween the masonry surface and graffiti, preventing graffiti from penetratinginto the masonry. They are also intended to make graffiti removal easiersince most graffiti does not adhere well to them. Generally, graffiti appliedover transparent barrier coatings can be removed with low-pressure waterand a detergent, or with a solvent.
There are basically two kinds of transparent barrier coatings: temporaryand permanent. Temporary, or "sacrificial" coatings are removedwhen graffiti is removed and then must be reapplied. Permanent transparentbarrier coatings are more resistant to the water or solvents used to removegraffiti, 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 permanentcoatings, 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 thesacrificial 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 offafter multiple cleanings, and must also be reapplied.)
Unfortunately, in application, there are a number of negative aspectsof transparent barrier coatings that generally prevent their being recommendedfor 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 lightingconditions or when it rains. Second, clear coatings may reduce the water-vaporpermeability of the masonry, thereby contributing to possible water-relateddeterioration. Third, the coating may discolor and change over time. Exposureto ultra-violet light can cause a coating to yellow; dirt build-up maydarken the treated surface; and some coatings acquire a sheen when rubbedor brushed against. Such changes are especially noticeable when only aportion of the building has been coated. Furthermore, if coatings are notmaintained 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 characterof the historic building.
Despite these potential drawbacks, there may be some instances in whichthe graffiti problem or frequency of occurrence is so severe that applicationof a transparent barrier coating on historic masonry may be worth considering.Some water-base polysaccharide coatings, and silicone and silicone-basecoatings have been used with success on masonry structures. They are essentiallyinvisible, and do not change the natural appearance of the masonry. Althoughless durable than solvent-borne coatings, they are water-vapor permeable(breathable), and may be reapplied to the masonry surface immediately afterremoving graffiti, while the surface is still damp.
However, extreme caution must be exercised before applying a transparentbarrier coating. Experimental test applications should always be triedfirst on discrete areas that are not highly visible, and the treated areasevaluated over a period of time. Laboratory test results on the performanceof coatings applied to samples of like masonry types may be useful to someextent. 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-sitewhere the factors of weather and pollution are the same as those at thelocation 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 thepotential for disfigurement, owners of landmark-designated buildings arerequired by some preservation review boards and landmark commissions toobtain 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 facilitatesthe removal of graffiti because graffiti does not adhere well to it. Pigmentedbarrier coatings that are water-vapor permeable may sometimes be used asa permanent barrier coating on non-historic masonry where thereis frequent recurrence of graffiti, and when constant surveillance is notpossible (Fig. 10). Although there are some instances in which pigmentedbarrier coatings may be appropriate on painted historic masonry, they arenot recommended for unpainted historic masonry because they willchange the appearance of the masonry. There is also another kind of pigmentedcoating that is specially formulated to be used as a temporary measureto 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 permanentmeasure 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. Anotherdisadvantage of using paint or a pigmented coating to hide graffiti isthat it usually appears as an obvious patch on unpainted masonry and tendsto attract more graffiti unless the paint can be applied in a discrete,and well-defined area (Fig. 11). If incompatible with either the masonryor the graffiti, such a coating may peel off the masonry surface in anunsightly manner. Like transparent coatings, pigmented coatings may bedifficult or impossible to remove completely once their performance orappearance is no longer satisfactory.
