PRESERVATION BRIEF: 38

REMOVING GRAFFITI FROM HISTORIC MASONRY


Martine E. Weaver

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service. 
To obtain a complete copy of this brief, including figures and
illustrations, please contact:  

               Superintendent of Documents
               P.O. Box 371954
               Pittsburgh, PA  15250-7954

               GPO #024-005-01121-8
               $1.50

Please call the Publication Order Information Desk at 202/783-3238
or FAX 202/512-2250 to verify price and availability.  


THE SECRETARY OF THE INTERIOR'S "STANDARDS FOR HISTORIC
PRESERVATION PROJECTS" EMBODY TWO IMPORTANT GOALS: 1) THE
PRESERVATION OF HISTORIC MATERIALS AND, 2) THE PRESERVATION OF A
BUILDING'S DISTINGUISHING CHARACTER.  EVERY OLD BUILDING IS UNIQUE,
WITH ITS OWN IDENTITY AND ITS OWN DISTINCTIVE CHARACTER.  CHARACTER
REFERS TO ALL THOSE VISUAL ASPECTS AND PHYSICAL FEATURES THAT
COMPRISE THE APPEARANCE OF EVERY HISTORIC BUILDING.


***INTRODUCTION***

Removing graffiti as soon as it appears is the key to its
elimination -- AND recurrence. Thus, the intent of this
Preservation Brief is to help owners and managers of historic
masonry structures find the best way to remove exterior, surface-
applied graffiti  quickly, effectively, and safely. The Brief will
discuss the variety of materials used to apply graffiti, and offer
guidance on how to remove graffiti from all types of historic
masonry without harming either the surface or the substrate.
Suggestions will also be given regarding the use of physical
barriers to protect masonry surfaces from graffiti, and the
application of barrier coatings to facilitate graffiti removal.
Building managers and owners of historic properties will be advised
on the importance of being prepared for rapid graffiti removal by
testing different cleaning techniques in advance in order to select
the most appropriate and sensitive cleaning technique. Health and
safety and environmental concerns are addressed, as well as
regulatory matters. Removing graffiti without causing damage to
historic masonry is a job for trained maintenance crews, and in
some cases, professional conservators, and generally should not be
attempted by untrained workers, property owners or building
managers. Although the focus of this Preservation Brief is on
historic masonry, the same guidance may be applied equally to
removing graffiti from non-historic masonry.


***IDENTIFYING THE GRAFFITI AND THE MASONRY***

Successful graffiti removal from historic masonry depends on
achieving a balance between breaking the bond between the graffiti
and the masonry surface without damaging the masonry. This
generally requires knowledge both of the materials used to make the
graffiti and the masonry on which the graffiti has been executed,
as well as knowledge of cleaning methods and materials. Without
this, masonry surfaces can be badly disfigured or damaged during
graffiti removal.

GRAFFITI. Most graffiti is made with spray paints. Although a
number of solvents and paint strippers are capable of dissolving or
breaking down these paints, some may permanently discolor or stain
the masonry surface if not used correctly. As a result, the
remaining paint may become more difficult, or even impossible, to
remove. Poorly thought-out and generally hasty attempts to remove
graffiti using harsh chemicals or abrasives can also cause
permanent damage to the masonry that may be worse than the
graffiti.

The ability to identify the graffiti material is an important step
in successful removal. Numerous kinds of spray paint
(polyurethanes, lacquers, and enamels), and brush-applied paints
(oils and synthetic resins such as vinyls, acrylics, acetates,
methacrylates, or alkyds), as well as permanent felt markers are
the materials most often used to make graffiti. But other materials
are also used for graffiti, including water-soluble felt markers,
ballpoint pens, chalk, graphite and colored pencils, pastels, wax
and oil crayons, liquid shoe polish and lipstick. The range of
materials adopted by graffitists continues to expand.

Paints are composed of pigments that provide color and hiding
power; binder that holds the pigments together and to the
substrate; and a solvent that allows the pigment/binder mixture to
flow. Some spray paints and markers may contain dyes instead of
pigments. Paints are applied wet. Generally, as the solvent
evaporates, the binder solidifies. The greater the solvent content
of the paint, the greater the flow rate, and thus, the greater the
ability of the paint to penetrate into masonry pores.

The two primary components contained in most graffiti materials --
pigment or dye, and binder -- may simply remain on the masonry
surface, or penetrate into the masonry to varying depths depending
on a number of factors, including the surface tension of the
substrate and viscosity of the solvent or vehicle. Thus, even the
total removal of the pigment or the binder may leave residues of
the other component actually in, or below, the surface of the
stone. Residual stains, or graffiti "ghosts," such as those from
any kind of red paint or the fine black pigments used in spray
paints, may be particularly difficult to remove. With painted
graffiti, it is helpful to establish how long it has been on the
surface. For most paints that have been on the surface for several
weeks or months, hardening processes are likely to be complete or
well-advanced; the solubility of the paint is proportionately
reduced and it will be more difficult to remove.

MASONRY. The historic masonry substrate must also be identified. As
used here, the term masonry encompasses all types of natural
stones; manufactured clay materials, including brick and terra
cotta; and cementitious materials, such as cast stone, concrete and
mortar. The common factor among masonry materials is that they are
porous, to a greater or lesser extent, and sensitive to abrasion.
After identifying the masonry, its condition, including fragility,
porosity and permeability, must also be assessed prior to beginning
graffiti removal. For example, a smooth, newly-polished granite
surface is comparatively easy to clean because it is relatively
impermeable and paint vehicles tend to stay on the surface rather
than penetrate into microscopic pores. A very smooth, polished
surface also has no pits or crevices that will retain particles of
pigment or binder. In contrast, weathered marble or limestone may
be extremely porous and permeable, with a rough surface on which
particles of pigment can easily lodge. The fragility of such a
surface can make it impossible to clean the surface even with a
bristle brush without risking severe surface loss. A difference in
surface texture or finish may also be the reason that a particular
cleaning agent will work in one situation but not another.

Some types of masonry may react adversely to contact with the
various cleaning agents required to break or dissolve the bond
between the graffiti and the masonry surface.

Thus, for purposes of cleaning, masonry types are often categorized
according to whether they are acid-sensitive, non-acid sensitive,
or alkali-sensitive. Acid-sensitive stones consisting of carbonate
materials may be damaged or even destroyed by contact with acids.
Although, in many instances, acidic cleaning compounds are not
effective for graffiti removal and generally should not be used for
this purpose, it is useful to know that some acid-sensitive
materials include: stones such as limestone, marble, travertine,
calcareous sandstones and shales; most polished stones, and glazed
architectural terra cotta and glazed brick. Non-acid sensitive
masonry materials include slate, granite, unglazed architectural
terra cotta and unglazed brick. Alkali-sensitive stones may contain
silicates, or ferrous, soluble iron compounds that can react with
alkalis or water to form severe staining. Alkali-sensitive stones
include some granites, Indiana limestone, and many types of
sandstone, especially those that are green or grey in color. Glazed
and polished surfaces tend to be damaged by both strong acids and
strong alkalis.


***GRAFFITI REMOVAL METHODS AND MATERIALS***

A variety of treatments are available from which to choose the most
appropriate method of graffiti removal that will not damage the
surface of historic masonry. Removal techniques, which are chosen
according to the type of graffiti and the masonry, range from
simply erasing pencilled graffiti with soft erasers, or removing
chalked graffiti with soft brushes, to poulticing with water (with
or without detergents), poulticing with water (with or without
detergents), poulticing with organic solvents or alkali-based paint
removers, or applying bleach to remove painted graffiti. In very
limited situations, it may mean using very delicate and controlled
abrasive means. Successful graffiti removal often requires a
combination of cleaning materials and methods.

POULTICING:

The most effective method of removing graffiti from masonry usually
involves the use of a poultice. A poultice consists of an absorbent
material or powder -- inert clays such as kaolin or sepiolite,
diatomaceous earth (fuller's earth); or cellulose products such as
fluff pulp cellulose or shredded paper -- mixed with a cleaning
solution (a liquid reagent such as water, organic solvent, paint
stripper or bleach) to form a paste or slurry. The purpose of a
poultice is twofold: it enables a cleaning solution to be kept in
contact with the stained area as long as possible, while allowing
the cleaning solution to pull the staining material out of the
substrate via the poultice without redepositing it in, or
restaining, the masonry. A poultice is often covered with a plastic
sheet to retard evaporation. With some extremely porous types of
stone, such as marble, although a poultice may remove a stain from
one side of the stone, stains can pass completely through the stone
and be redeposited on the other side of the masonry slab. Thus,
caution should always be exercised in stain and graffiti removal.

WATER AND DETERGENT. Graffiti removal from historic masonry should
always begin with the gentlest means possible. In some instances,
this means low-pressure water washing. Fresh graffiti -- one or two
days old -- made with water-soluble markers may sometimes be
removed with water, possibly aided by a neutral or non-ionic
detergent. (Non-ionic detergents which do not ionize in solution,
do not deposit a solid, visible residue.) Ammonia can also be
effective in removing fresh graffiti. Any detergent should be
approached with caution and tested before using because most
commercial laundry detergents are not neutral and contain
substances which may leave undesirable residues on masonry
materials. Usually, the water and detergent should be mixed with an
absorbent material and applied in the form of a poultice. Although
water washing is often likely to be the gentlest cleaning methods
for historic masonry, it may not be as effective for removing
graffiti because many graffiti materials are not soluble in water.

ORGANIC SOLVENTS AND PAINT REMOVERS. Most graffiti can be removed
without damaging the masonry with proprietary graffiti-removal
products and commercial paint strippers containing organic
solvents. But, these products should always be tested and used in
accordance with manufacturer's instructions included in the product
literature. Normally, solvents should be used in a poultice form to
prevent them from penetrating into the substrate, and permanently
discoloring or staining the masonry. A number of paint-removers are
manufactured as thick gels or pastes that cling to the surface, and
some commercial paint-removal products include a tough fiber-
reinforced paper or cloth backing that retards evaporation and also
facilitates neat and clean removal of the used stripper. The
advantage of using organic solvents is that they evaporate
completely, leaving no residual material in the masonry. However,
organic solvents may present a severe health hazard, and workers
using them must wear adequate protection. "Off-the-shelf" aerosol
graffiti removers generally should not be used because the
dissolved paint being removed may run down the wall 'staining' a
previously clean area; or pigments may also be redistributed by the
rinsing and scrubbing recommended by the product manufacturer.

ALKALINE COMPOUNDS. Alkaline compounds may be used to remove some
oils and greases, and waxes from non-alkali sensitive masonry. Like
organic solvents, alkaline compounds should generally be used in
conjunction with a poultice when removing graffiti. The use of
alkaline compounds should always be followed by a weak acid wash
and a water rinse in order to neutralize -- or remove -- all the
alkaline residues from the masonry. Strong alkalies (pH13-14), such
as sodium hydroxide-based paint removers (caustic soda or lye),
generally should not be used as they can cause efflorescence and
staining on masonry surfaces, if not properly neutralized.
Potassium and other hydroxide paint removers may react with iron
compounds in some masonry, particularly Indiana limestone, to form
dark brown (rust-colored), or black ferric hydroxide stains, which
are very difficult to remove.

BLEACHES. Alkali-based bleaches such as calcium hypochlorite can
sometimes be used very successfully in a poultice to bleach or
decolorize certain dyes contained in some paints and inks that
cannot readily be removed by other means.

MECHANICAL OR ABRASIVE METHODS. Mechanical treatments include dry
or wet blasting, using abrasive grits, such as sand, dolomite
powder, aluminum oxide, ground-walnut shells, sodium bicarbonate
(baking soda), and others; high-pressure water washing; and
mechanical sanding or grinding. All of these abrasive methods will
cause damage to masonry and, in most instances, should never be
considered as a method of removing graffiti from historic masonry.
Abrasive methods used mistakenly by untrained workers to remove
graffiti usually result in etching the outline of the graffiti
permanently into the masonry. Some historic masonry materials can
be easily damaged by pressure washing even at low or moderate
pressures (100-400 psi). Occasionally, however, under very
controlled circumstances, a micro-abrasive technique may be
appropriate for removing graffiti from delicate masonry surfaces,
if used at low pressures of 35-40 psi with fine abrasives. This
treatment, which must be done very slowly and carefully to avoid
damaging the masonry, should be tested first, and undertaken only
by a professional conservator. Another exception, even though it is
not strictly an abrasive treatment, is using a razor blade as a
first step to remove spray paint or felt-tip marker from polished
granite. However, this too, should be undertaken only by a
professional conservator, and only on polished granite, which is
very hard and generally impervious to scratches.

LASER CLEANING. Although not in general use as a cleaning
technique, laser technology offers great promise in the future as
a non-damaging method of graffiti removal.


***TESTING***

Before selecting a removal method, all cleaning materials and
techniques for removing graffiti from a historic masonry building
should be tested on mock-ups or areas of the resource that are not
highly visible, but which are representative of typical conditions.
Visual observation should be supplemented by the use of a
magnifying glass, and spot tests should be carried out with various
solvents to help identify the specific graffiti medium, which will
aid in its removal. More complex testing using laboratory equipment
and more scientific analytical processes may sometimes be necessary
in complex situations. Sample areas that represent the desired
degree of "cleanliness" should be approved in writing by client,
architect, conservator or other appropriate authority. The
materials and all the other data necessary to reproduce the desired
cleaning results should be meticulously recorded and the accepted
sample area preserved for reference until the end of the job. The
existence of a "clean" sample for comparison and a signed agreement
can avoid unpleasant surprises, misunderstandings, and perhaps
legal actions.

When a type of graffiti appears for the first time that was
executed with a material not immediately recognizable and for which
no countermeasures have been developed, tests may need to be
carried out by an architectural conservator to identify the
material and to determine effective removal treatments. Agencies
with large inventories of graffiti-prone buildings and structures
should watch for graffiti made with new materials and experiment
with different cleaning methods in order to be prepared when it
appears. Such early action can save large sums of money in the long
term.


***DEVELOPMENT OF A TREATMENT PLAN***

For managers or owners of historic masonry buildings, or agencies
responsible for large inventories of graffiti-prone properties,
including parks, highway and railroad bridges and viaducts, bus,
train and subway stations, and cemeteries, the development of a
treatment plan may be the first step toward an effective graffiti-
removal program. It is becoming increasingly common for large or
important historic properties to have regular maintenance and
disaster plans that include graffiti removal.

When feasible, a separate treatment plan should be prepared for
each structure. However, if this is not possible, it is advisable
to prepare a variety of treatment plans for specific masonry types.
Plans should be prepared to cover all types of masonry that fall
under one jurisdiction, management or ownership that are potential
targets for graffiti.

Guidance contained in treatment plans should be based on the
results of carefully controlled testing to remove a wide variety of
common graffiti materials safely, and without damaging the various
types of masonry. Individual treatment plans should address all
parts of the building or structure that could be disfigured by
graffiti, and any features too fragile to be cleaned by anyone
other than a conservator should be noted on the plan.

A treatment plan is essentially a cleaning specification, but it
should also include information on the following:

-    the types and conditions of masonry likely to be targeted by
     graffiti;
-    methods, materials and techniques known to work most
     successfully in the removal of specific types of graffiti from
     the surface of each type of masonry;
-    sources for materials;
-    a list of contractors with expertise in graffiti removal,
     including names, telephone numbers, information on emergency
     access to the property, and storage location of materials;
-    graffiti-removal methods which may be harmful to the masonry
     surface;
-    contractors or consultants who are not acceptable and should
     not be considered for graffiti removal;
-    scaffolding, pumps, or safety equipment that might be
     required, where it is available, and costs involved; and
-    health and safety concerns regarding specific removal
     treatments, product literature and Material Safety Data Sheets
     (MSDS).


***HEALTH AND SAFETY CONSIDERATIONS***

Most of the chemicals used for graffiti removal are dangerous to
workers, as well as to others who may be in the vicinity. Organic
solvents are toxic by ingestion, inhalation, and skin contact.
Material Safety Data Sheets (MSDS), available from the product
manufacturer for all paint-removal products, should always be
consulted and followed. Identification of hazardous components and
checking with chemical reference works will help assure that the
least hazardous, but most effective, products are selected.

Generally speaking, it is a sensible policy to carry out all
graffiti removal in well-ventilated conditions. Some solvents can
be used only outdoors, and sometimes forced ventilation may be
necessary even there, requiring workers to use air-fed respiratory
equipment to avoid wind-blown fumes. Smoking, eating or drinking
must not be allowed when cleaning is in progress.

Some materials used for graffiti removal are so corrosive that
accidental contact can cause serious, permanent scarring and
painful injuries. Wearing appropriate protective clothing must be
strictly enforced. Mandatory personal protective equipment (PPE)
normally includes face shields or safety glasses; long, chemical-
resistant gloves; face masks with respirators for organic solvents;
and possibly, full protective clothing with an independent air
supply.

All smoking and open flames should be rigorously excluded from work
areas; many solvents are flammable or highly explosive in vapor or
liquid form when mixed with air. Solvent residue, used swabs,
cloths, overalls and all other solvent-contaminated items should be
safely and legally disposed of, or properly stored -- even
overnight -- away from potential sources of fire. Electrical
equipment may require explosion-proof fittings when used with
certain solvents.

When electric pumps and pressure-spraying equipment are used, it is
especially important that all necessary precautions be taken to
avoid electric shock. Water sprays and puddles on the ground
present a potentially dangerous situation, if they come into
contact with temporary wiring at work sites where graffiti is being
removed. Such hazards must be carefully monitored and controlled.

As with any construction project, attention should always be
directed toward the general safety of the workers and passers-by,
but also toward possible damage to the resource itself that might
result from careless placement of ladders, or scaffolding.
Chemicals used for masonry cleaning can also damage adjacent
metals, glass, and painted surfaces, as well as vegetation. Product
manufacturers' instructions should always be closely followed to
avoid such inadvertent 'collateral' damage.


***ENVIRONMENTAL CONSIDERATIONS***

To protect against environmental contamination, including the
formation of unwanted ozone at ground level and damage to the ozone
layer in the earth's outer atmosphere, legislation has been enacted
in some states making it illegal to use even moderate quantities of
some solvents -- volatile organic compounds (V.C.) contained in
paint removers. In response to this legislation, many new products
are being developed that do not contain VOC.

After completing graffiti removal, the disposal of chemical
products and rinsing effluent must be taken into account.
Arrangement for disposal of the cleaning waste should be made prior
to beginning graffiti removal, especially if it is a project of
considerable size. In many places it is illegal to discharge
solvents and/or paint residues into sewers or storm drains. The
owner or manager of a historic property, or in some cases the
individual or firm doing the cleaning or graffiti removal, is
responsible for being informed of, and complying with, relevant
laws and regulations. Under provisions of the National Historic
Preservation Act of 1966, as amended, approval may be required from
a state or Federal preservation agency before any work can be
undertaken on buildings or structures listed in or eligible for
listing in the National Register of Historic Places, if such a
project involves Federal funding or licensing. Many state and local
historic district commissions and review boards have their own
regulations that require approval for cleaning or graffiti removal
work that is undertaken on landmarks or properties in locally
designated historic districts.


***BARRIER COATINGS***

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 use of barrier coatings to protect
graffiti-prone historic masonry surfaces may 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 (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-based polysaccharide
coatings, and silicone and silicone-based 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
transparent barrier 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
sample 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, and the use of a barrier coating is
determined necessary, an architectural conservator should evaluate
the test performance of a variety of coatings before 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 on
masonry as a permanent, preventive barrier coating, or as a
temporary means 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 as a permanent barrier coating on
non-historic masonry where there is frequent recurrence of
graffiti, and when constant surveillance is not possible. 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 especially 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 felt markers, bleed through the coating; and repeated
applications of the coating or paint can result in a heavy paint
build-up on masonry surface. Another disadvantage of using paint or
pigmented coating to hide graffiti is that it usually appears as an
obvious patch on unpainted masonry and tend to attract more
graffiti unless the paint can be applied in a discrete, and well-
defined area. 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
be difficult or impossible to remove completely once their
performance or appearance is no longer satisfactory.


***CRITERIA TO CONSIDER BEFORE SELECTING A BARRIER COATING AS THE
PRIMARY PROTECTIVE MEANS OF COMBATING GRAFFITI***

What to look for in a Barrier Coating:

-    Water-vapor permeable, or "breathable."
-    "Invisible" without gloss or sheen, when applied to masonry.
-    No change in appearance from uncoated areas when masonry is
     wet.
-    Does not discolor or attract dirt.
-    Weathers evenly.

Questions to Ask:

-    Will the coating last long enough to offset its cost?
-    Will the application and reapplication of the coating be cost
     effective?
-    Will the coating be effective against more than one type of
     graffiti?
-    Can the coating be completely and thoroughly removed, so that,
     if necessary, paint, or another coating will adhere to the
     masonry surface?
-    Will the building ever need to be repointed or patched? A