 The difference in color between the bottom and the top of the stone spandrel is the only clue to the presence of a clear barrier coating. Photo: NPS files. |
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 (although
this 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 first
applied to the masonry surface, after which a sacrificial layer consisting
of 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 a
gloss 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 a
portion 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 which
the 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.
"Removing Graffiti from Historic Masonry" an Historic Preservation Brief
