Striking Out Graffiti
To select the right coating for the job, consider the level of protection, cleaning method and frequency level you need, as well as the long-term cost.Far from a mere eyesore, graffiti has serious costs. Many cities suffer from some amount of graffiti, devoting substantial resources to prevent or at least remove it. Besides being unsightly, graffiti can intimidate a community’s residents. By creating the appearance of a run down or dangerous area, it can scare away customers and potential residents. Patrons of buildings, parks, transit system s or public facilities where graffiti vandalism has occurred may feel that more serious crimes also may go unchecked .The cost of prevent ing and removing graffiti to U.S. taxpayers and private property owners is in the billions. Simply painting over graffiti is one common solution. Sometimes the tags will bleed through and shadow (or “ghost”) the new paint, resulting in a patchwork appearance of mismatched colors and textures. Worse, a freshly repainted surface often proves irresistible to taggers, both perpetuating and accelerating the cycle. The good news is that the market offers a range of graffiti-resistant coatings, films and other technologies to help protect various exterior and interior surfaces.
The cities themselves have developed a number of protocols using graffiti-removal cleaners. Chicago’s model Graffiti Removal Program, for instance, employs “blast” trucks that use baking soda under high water pressure to remove painted graffiti from brick, stone and other mineral surfaces. (The program also deploys paint trucks to cover graffiti on surfaces such as metal or wood.) However, repeated cleaning with mechanical means of removal can be destructive.
Chemical graffiti removers have varying degrees of effectiveness and substrate compatibility. Cleaned surfaces often have noticeable shadowing or discoloration, with porous surfaces such as concrete, masonry and stone being especially problematic. In most cases, chemical strippers and solvents will drive graffiti stains deeper into the substrate.
The key to using anti-graffiti surface treatments or coatings is to first evaluate the surface to be protected, as not all products are suitable for all substrates or circumstances. For example, how porous is the surface? Has the surface already been tagged, painted or cleaned, or coated with another anti-graffiti product? Then consider other factors: How accessible is the surface to coating and cleaning operations? Will the product alter the surface appearance? How likely or with what frequency is the tagging? Let’s examine the protection strategies with these questions in mind.
Assessing the Coatings Options
A wide variety of graffiti-resistant treatments and coatings is available, but they generally fall into two main categories: sacrificial and non-sacrificial. Sacrificial coatings, true to their name, are sacrificed along with the tag during the removal process and must be reapplied to continue protection. Non-sacrificial, or permanent, anti-graffiti products prevent graffiti from adhering to the substrate. Sacrificial products can be applied to bare substrates or over other types of coatings. They usually are single-component systems based on silicone or wax, and usually they are water-based. Generally they are removed using hot water in a pressure washer. Therefore, the availability of utilities is a consideration. One or two coats are generally required. The cost of these products is low, but the overall economics will be dictated by the size and frequency of tags to be removed and how often the product must be reapplied.
Semi-sacrificial (sometimes called semi-permanent) coatings have a moderate resistance to aggressive cleaning methods. These are
high-build products, generally acrylics. They eventually require reapplication after a certain number of cleanings, as each removes some of the product surface; some advise recoating after every second cleaning. The substrate should be recoated before all of the coatings erode or they may not remain effective. Non-sacrificial permanent products may be either film-forming or non-film formats. The latter are a relatively newer product category and generally work by chemically bonding with and modifying the free surface energy of the substrate to reduce or prevent the adhesion of any graffiti. The film formers include those based on polyurethanes, nano particles, fluorinated hydrocarbons or siloxanes and are available as both one-and
The graffiti usually is easily removed, without shadowing, by pressure washing with hot water, although some products may require a solvent based cleaner. Non-sacrificial products are especially useful for mural-sized tags or where tagging and removal are frequent. These coatings tend to cost more than sacrificial coatings, but they may need to be applied only once.
When it comes to single-pack offerings, both solvent-borne enamels and water-based acrylics are available. However these are not nearly as good as the higher performance two component systems in resisting solvent-based tagging or removal agents. That’s because the graffiti solvent can soften the coating, allowing dyes to stain and shadow the coating.
Permanent two-component coatings do form a protective, highly cross linked, low porosity hard film over the substrate. Solvent-borne polyurethane formulations are widely used, and most may be pigmented to cover existing tags or used as a clearcoat.
With tints, neutral colors such as concrete gray and sandstone will show less staining and appearance change than lighter or higher chroma colors, making them better suited for areas that are frequently attacked.
The two-component products usually have a satin to glossy sheen, and in general, the higher the level of gloss, the more effective the graffiti resistance and removal properties will be. In some cases it is important to match the gloss of the substrate, like the satin finish of most bridges. In other cases, gloss must be kept low, to avoid the glare from headlights, for example. Some formulations, such as some two-component polyurethanes, can be formulated in low-gloss versions.
Regarding appearance, the two-component products can darken surfaces such as stone or concrete in a “wet” look. This appearance difference typically is an issue only when the coating is applied to part of the wall, say, only within tagging reach. However, for building facades where a natural stone or concrete look is desired, the owner may prefer a non-film forming surface treatment. With the two packs, aggressive, product-specific specialty cleaners are usually required. These cleaners dissolve everything but the coating, so always consult the manufacturer’s spec sheet for requirements when selecting a product.
With all products, the sooner the cleaning is begun, the better is the effectiveness of removal, as the tag will usually continue to cross link and harden or penetrate the coating or substrate. Also, “tags attract tags,” leading to more to remove.
With two-component products, the coating will further cross link with age, increasing its graffiti-resistance . If possible, protect the two-component products from graffiti until they have fully cured. Most of the single-component products will age from UV and weather exposure and gradually lose their effectiveness. The rate of degradation may depend on the overall climate and even which compass direction the product faces. For products exposed to direct sunlight, consider those with UV stabilizers.
Testing Their Effectiveness
The type of tagging agent used, such as solvent based permanent markers or spray-can paints, will also determine the degree of difficulty of removal. Higher-grade products within a generic formulation type will employ higher-quality raw materials, resulting in better performance. So what tests are available to help gauge the effectiveness of these products against vandalism?
One standard is ASTM 06578/06578M-13 Standard Practice for Determination of Graffiti Resistance. In this test, several marking agents are applied to smooth metal test panels that have been treated with the product. The standard marking agents used include a solvent-based permanent blue-ink marker, acrylic and alkyd red spray paints, a blue or black wax crayon, ballpoint ink and a water based black-ink marker. The test recommends that the surface be marked and cleaned both before and after natural or laboratory-accelerated weathering (by UVA- 340 fluorescent or daylight-filtered xenon arc instruments) for comparison. Standard cleaning agents (for products without their own removers) include dry cloth, mild detergent, isopropyl alcohol, mineral spirits, xylene and methyl ether ketone (MEK); other agents may also be used. The degree of removal and any residual marking is qualitatively evaluated visually or instrumentally for changes in color or gloss and reported. An optional procedure tests panel re-cleanability for non-sacrificial products.
For porous surfaces, we can use ASTM 07089-06 Standard Practice for Determination of the Effectiveness of Anti-Graffiti Coating for Use on Concrete, Masonry and Natural Stone Surfaces by Pressure Washing. This method applies only to unweathered specimens of tagging agents applied to protected concrete, natural stone and masonry substrates. The marking agents used are all solvent-based: spray paints and permanent-ink markers in colors of black, blue and red. Cleaning is performed by high-pressure sprays of cold and hot water and sodium bicarbonate. As in
06578, recleanability can also be evaluated. However, laboratory tests on small samples under controlled conditions may not always match actual field performance.
Despite the frequent graffiti tagging of infrastructure such as concrete sound walls, bridges and highway signage, most U.S. state departments of transportation (DOTs) do not regularly use anti-graffiti products. That is due to a combination of factors, including the cost of materials applied to very large structures and durability and performance, especially on porous concrete. Instead, they tend to opt for paint overs or blast cleaning. However, some DOTs do perform field trials, or require their own tests, to evaluate products when they want to protect high-value or more limited targets. One example is California’s specification for testing anti graffiti coatings and films for retroreflective highway signs (California Test 684). Similarly, the Texas DOT has a comprehensive test method, Tex- 890-8 Determining the Graffiti Resistance of Anti-Graffiti Coatings. Texas requires the product to be applied to cementitious mortar panels, aged in a xenon arc Weather-Ometer, and then marked with a sequence of aggressive paints and markers and cleaned. Like Texas, the Nordic countries have developed a performance test for wax sacrificial coatings on various mineral surfaces, including stone, masonry and concrete. As in the Texas method, the applied coating is pre-aged in a xenon arc Weather-Ometer before marking and then instrumentally evaluated for graffiti removal and changes to the substrate color and gloss. The Rhode Island DOT conducted both laboratory tests and field trials of several permanent products and found generally good performance, but noted that performance varied. Rhode Island also noted that the permanent products were cost-prohibitive for large areas, but were recommended “in areas prone to acute graffiti vandalism.” Again, however, protecting a high-value building facade or other surface is substantially different from protecting miles of concrete barriers or bridges.
One of the more comprehensive studies of non-sacrificial anti-graffiti coatings was a field evaluation commissioned in 2009 by the Nevada DOT (NDOT) with the University of Nevada-Las Vegas. Here, a dozen suppliers submitted 14 products or systems and applied them to three types of challenging concrete substrates. These substrates included both painted and unpainted concrete panels and a soundwall with fractured fin facing. The manufacturers applied a representative variety of one and two-component products of various chemistries in early fall for the purpose of developing DDT specifications. After curing, aerosol spray paint was applied in stripes of red, pink, green and black; after one week, the manufacturers removed the stripes. A panel of DDT personnel evaluated the removal effort, effectiveness and other parameters, such as damage to substrate. Testers then conducted durability testing through the repeated application of graffiti and re-cleaning, as experience had shown that some “permanent” products were sub-par performers.
NDOT determined that field evaluation is the most feasible method for qualifying non-sacrificial, anti-graffiti coating products. Of the 14 products tested, two were disqualified for nontechnical reasons. Of the remaining 12, four systems were recommended for further detailed evaluation. Eight systems showed varying degrees of effectiveness in graffiti resistance, cleaning, surface damage, residue or appearance such as gloss on one or more
of the substrates.
Looking Ahead to New Technologies
The chemistry for anti-graffiti coatings continues to evolve one step ahead of the taggers, providing ever more effective, and affordable, formulations. For example, we now see penetrating silicone-based treatments for concrete that are essentially permanent but cleanable with low pressure water spray. Newer nano-technology based products now emerging bond with the substrate to form a ceramic or glassy surface that resists marking and substrate penetration. Another development, nano-scale transparent titanium dioxide technology used for self-cleaning surfaces, also shows photolytic activity for destroying the organic components of marking agents.
Also, not to be forgotten, factory-applied graffiti-resistant powder coatings are available for both interior and exterior applications such as building panels. And yet another technology may be an alternative where a coatings solution isn’t feasible: the use of anti-graffiti protective films. These are usually transparent products and may be applied to optical surfaces such as retroreflective signs and glass windows.
Various clear and colored anti-graffiti coatings are available to protect private and public assets from the urban scourge of graffiti. The choice of protection strategy and specific product is a balance between protection, cleaning method and effort, frequency and asset value of the target, aesthetics, substrate compatibility and, finally, product and application costs. Although “one size fits all” doesn’t apply to anti graffiti coatings, by using the proper product, you can keep one step ahead of the tagger.
About the Author
Allen Zielnik is senior consultant-weathering science in Atlas Material Testing Technology LLC’s global Consulting Solutions group, where he specializes in the weather durability testing of materials and products. A frequent speaker at various worldwide technical symposia, he is the author of more than
120 publications and conference presentations. He has degrees in electronics engineering and analytical chemistry, and since 1994, he has been involved with natural and accelerated weathering technology and solar simulation with Atlas Material Testing Technology. D+D