Effective Use of Roof Coatings

EFFECTIVE USE OF ROOF COATINGS
MATTHEW M. COPELAND, GREGORY R. DOELP,
AND PAUL C. SCHEINER
SIMPSON GUMPERTZ AND HEGER
41 Seyon Street, Building 1, Suite 500, Waltham, MA 02453
Phone: 781-907-9000 • Fax: 781-907-9009 • E-mail: mmcopeland@sgh.com and grdoelp@sgh.com
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ABSTRACT
Roof coating manufacturers attribute many benefits to their roof coating systems,
including protection of underlying roofing from damaging ultraviolet rays, reduced thermal
cycling of the roof, improved waterproofing performance, and, with reflective coatings,
reduced energy costs. This paper summarizes the major coating types available including
acrylics, polyurethanes, and specialty products such as soy-based coatings, and discusses
the applications that each type is best suited for. The authors explain the importance of
identification of suitable roofs and suitable coating products for those roofs, coating characteristics
important for successful coating performance, and appropriate performance and
maintenance expectations for coating systems.
SPEAKERS
MATTHEW M. COPELAND — SIMPSON GUMPERTZ AND HEGER WALTHAM,
MA
MATTHEW M. COPELAND joined Simpson Gumpertz & Heger Inc. (SGH) in 2005 after
receiving a BS in civil engineering from Northeastern University. He is a project engineer
specializing in historic building investigation and repair projects, with a focus on materials
science issues. He has experience with a variety of building envelope issues including implementation
of innovative technologies, such as building-integrated photovoltaics and custom
terra cotta rain screen systems. Mr. Copeland is a registered professional engineer in the
Commonwealth of Massachusetts.
GREGORY R. DOELP — SIMPSON GUMPERTZ AND HEGER WALTHAM,
MA
GREGORY R. DOELP joined Simpson Gumpertz & Heger Inc. (SGH) in 1984 with a BS
in civil engineering from the University of Delaware and a MS in Civil Engineering from
Cornell University. He specializes in investigating and designing roofing, plaza waterproofing,
and below-grade waterproofing systems. His projects have also included analyzing
building moisture problems and leakage problems, as well as repairing and renovating
structures. Mr. Doelp is a member of the American Society of Civil Engineers and several
honor societies. Mr. Doelp is a registered professional engineer in the Commonwealth of
Massachusetts and seven other states.
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Binders Resins)
EFFECTIVE USE OF ROOF COATINGS
INTRODUCTION
Roof coating manufacturers attribute
many benefits to their liquid-applied roof
coating systems, including prolonging the
service life of the existing roof system, protecting
the underlying roofing system from
damaging ultraviolet rays, reducing thermal
cycling of the roof, repairing minor leaks
and cracks to restore waterproofing performance,
reducing building energy costs, and
improving building aesthetics. Some manufacturers
offer watertightness warranties for
their coating systems.
With so many advertised benefits, many
building owners are eager to embrace the
promises of roof coatings as an inexpensive
way to address their roofing problems. For
example, simply coating over an existing
aging built-up roof sounds like an attractive
alternative when presented to a building
owner previously faced with a choice of
either an ongoing maintenance regimen of
leak chasing and patching or a wholesale
roof replacement. A coating project is relatively
inexpensive and noninvasive, which
can be particularly important for occupied
buildings. The finished product often looks
like a new roof when first installed, but a
deeper look is necessary to see if a roof coating
will live up to its claims.
When it comes to real-world performance,
roof coatings certainly have their
place in the roofing industry and have been
used successfully on many buildings, but
they are not a cure-all for every old roof
approaching the end of its service life. Many
factors must be taken into account by the
building owner and the designer when
determining whether a roof coating is
appropriate for a particular building. The
capabilities and limitations of the roof coating
must be consistent with the owner’s
expectations for the performance of the roof
coating. Once it is determined that a particular
roof is appropriate for a roof coating
application, the correct coating for that particular
application must be selected in order
for a project to be successful.
Many roofing systems are candidates for
roof coatings, including metal roofs, builtup
roofs, single-ply membranes (PVC,
EPDM, etc.), and modified bitumen. Some
roofs are better suited for coating application
than others, and the condition of both
the exposed roof surface and the underlying
materials is key to a successful application
of a liquid-applied coating.
COATING BASICS
Roof coatings, like other coating systems,
are made up of several constituents,
including a solvent (sometimes also known
as the carrier), a binder (sometimes also
known as resin), pigments, and other additives.
Coatings are often classified or
referred to by the type of solvent or binder
used in the formulation. These components
play an important role in determining a
coating’s physical characteristics and performance,
so it is helpful to have at least a
rudimentary understanding of coating composition
when discussing coating performance.
(
The most critical coating component is
the binder, also known as the resin, which
is the film-forming component of the
coating. The binder is the component
that has the most influence on the
coating’s physical characteristics
such as elongation, water absorption,
tensile strength, and the coating’s
adhesion properties. Coatings are
manufactured with a wide variety of
binders; some common binders
include acrylics, polyurethanes,
alkyds, silicones, and polyvinyl
acetates. The type of binder is often
combined with the type of solvent
(discussed below) when referring to a
particular coating, i.e., “water-based
acrylic.”
Acrylic binders are common in
water-based roof coatings. These
water-based acrylic coatings make up
a large portion of the systems marketed
as white “cool roof” coatings. These
water-based formulations are generally
designed to meet local volatile
organic compound (VOC) regulations;
ics also exist. Coatings based on acrylic
binders are often less expensive than coatings
based on other binders.
Though polyurethane-based coatings
are generally more expensive than acrylics,
they often offer better long-term performance,
particularly when exposed to ponding
water conditions. Polyurethane can be a
one- or two-component product, with twocomponent
products typically curing more
rapidly but requiring greater applicator skill
to install correctly. Aliphatic polyurethanes
are more stable than aromatic compounds
when exposed to ultraviolet light, are therefore
less prone to yellowing, and are typically
used in topcoats of polyurethane roofcoating
systems.
There are now many specialty coatings
currently on the market, purporting to provide
performance similar to more traditional
coating products while using alternative
materials. Such coatings are being introduced
in part because of various green and
sustainability initiatives. Among the most
popular of these products are coatings containing
soy products. The soy components
The terms drying and curing refer to
different processes, but if they are used
interchangeably, some confusion can
result. Drying simply refers to the
evaporation of the coating solvent, leaving
the binder and other components behind.
Curing describes the actual chemical
process by which the binder forms a film
over the substrate. In some cases, the film
is formed simply by evaporation (drying) of
the solvent; however, most often, the film
is formed by a chemical reaction within
the binder. Various mechanisms of
reaction exist, and the curing mechanism
must be considered when evaluating a
coating for a particular use.
though organic, solvent-based acryl- Figure 1
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Solvent Carrier)
Water-Based Coatings
Solvent-Based Coatings
Percentage Increase After Water
Absorption (ASTM D471)
Coating Sample
Average Linear
Dimension in Plane Mass Thickness
Water-based acrylic 1 3.1 10.4 10.0
Water-based acrylic 1 (reinforced) 1.6 3.2 3.1
Water-based acrylic 2 2.1 12.5 10.5
Water-based acrylic 3 0.0 3.2 6.3
Water-based acrylic 4 2.6 11.1 12.6
Solvent-based soy alkyd 1 4.2 13.5 11.7
Solvent-based soy alkyd 2 1.8 6.6 NA*
Solvent-based aromatic polyurethane 1.5 3.5 1.2
*Data not available.
Figure 2 – Water absorption properties of selected coatings after one year of
outside exposure.
of these coatings are implemented in a variety
of ways, such as modified-polyurethane
and alkyd binders, but the coatings are generally
marketed as “soy-based” regardless of
the underlying chemistry, simply because a
portion of their polymers are derived from
soybeans.
We performed laboratory testing on several
roof-coating products. We generally
found that some soy-based coatings provide
physical properties within the same range
as their nonsoy counterparts (Figure 2).
However, simply selecting a soy-based coating
is not an assurance of similar performance;
the principles described below must
still be followed.
(
The solvent (also sometimes known as
the carrier) in a liquid coating makes the
application of the coating possible and is
not a part of the final cured coating film.
The other components of the coating are
suspended in the solvent, which typically is
water or a VOC such as xylene or toluene.
For this reason, coatings are commonly
referred to as “water-based” or “solventbased.”
Some coating formulations are
designed without a solvent to avoid the
issues of VOCs and loss of solvent during
curing. These are referred to as “high solids
coatings” or “100% solids coatings.”
Water-based coatings dry by evaporation
of the water. However, water-based
coatings cure by coalescence of the emulsion
particles in the binder into a uniform
film (the difference between curing and drying
is discussed in the sidebar, Figure 1).
Coalescence is a process involving controlled
evaporation of water and cosolvents,
which allows emulsion particles to come
into close contact with one another and
react, forming the cross-linked coating.
Water-based roof coatings are less sensitive
to application over damp substrates than
are solvent-based coatings because small
amounts of water can diffuse through the
coating surface. Although the substrate
water may not affect
the curing of the liquid-
applied coating,
the substrate water
may inhibit bond of
the coating to the
substrate.
Application of
water-based coatings
is typically limited to
ambient and substrate
temperatures
greater than approximately
40 to 45ºF
because of concern
that condensation
may occur on the
substrate and inhibit
bond or that condensation
may form on
the freshly applied
coating, slowing the
cure and washing
out the binder. Rain
shortly after application
of water-based
coatings can also cause similar problems.
In addition, high ambient humidity can be
detrimental to proper coating curing as the
high humidity slows the rate of water evaporation.
Despite these limitations, waterbased
coatings are popular because they
are easy to apply, and the equipment can be
cleaned with water alone.
After application, solvent evaporates out
of solvent-based coatings during the drying
process and initiates the curing. The loss of
solvent concentrates the binder molecules
so they can react to form the coating film.
The rate of evaporation is influenced by
temperature; solvent evaporates more
quickly in warmer temperatures. If the solvent
evaporates too quickly or too slowly,
the polymerization reactions of the resin
may not occur effectively, and the resin may
not form an effective film. If the substrate is
damp, the binder may not adhere properly
to the substrate. Moisture may interfere
with the bond of the coating, and moisture
vapor trying to escape from the substrate
may break the adhesion of the new coating
while it is still weak, so that an adequate
bond never develops. A general rule of
thumb is that solvent-based coatings
should be applied only when the temperature
is 5ºF above the dewpoint and rising,
but less than approximately 95ºF. Similar
The terms latex and elastomeric are often used when
describing coating systems. Sometimes these terms are
used in a manner similar to terms like “acrylic” or
“polyurethane,” and if so, they may create some confusion
because they are thought to describe some type of coating
binder. Technically speaking, latex refers to an emulsion,
but it is also used to refer to a rubber compound that can
be naturally occurring or synthetic. In the context of roof
coatings, the term latex does not describe a specific binder
polymer but instead refers simply to a water-based coating
that can contain a variety of different binder polymers.
Likewise, the term elastomeric does not refer to a specific
type of coating at all, but rather a property of a coating.
Many different coatings have elastomeric properties that
are defined by the coating’s ability to stretch considerably
and then return to its original shape.
Figure 3
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Determine Whether a Roof Is Suitable
for Coating
Adequate attachment.
Sound roof substrate.
Freedom from ongoing major
leakage issues.
Freedom from moisture in the
roofing system.
Adequate slope-to-drain.
to water-based coatings, if the freshly
applied coating surface becomes damp with
condensate or rain, portions of the resin
may wash out or become diluted, resulting
in an incompletely cured coating. Solventbased
coatings generally are less permeable
and develop better adhesion to a given substrate
than water-based coatings.
The use of solvents is often limited by
VOC regulations imposed by federal, state,
and local jurisdictions. A desire to limit
VOC content is often the cause of product
formulation changes from solvent-based to
water-based or to high-solids formulations.
Issues with solvent-based coatings include
flammability of the solvent and the potential
for the vapors to enter the building ventilation
system; odors from the coating affecting
building occupants, even if the building
ventilation system is isolated; and health
concerns from exposure to the vapors and
the coating itself. Solvent-based coatings
may be incompatible with other roof-system
components in contact with the coating,
such as the existing roof membrane, flashings,
or sealants, which may be soluble in
the solvent. Solvent-based coatings typically
require the use of solvents to clean tools.
See Figure 3.
PRINCIPLES FOR SUCCESSFUL
ROOF COATING APPLICATION
Successful roof coating applications
depend on a specific process, beginning
with evaluating whether a particular roof is
suitable for coating and then identifying an
appropriate type of coating for the application.
The general process is outlined below.
Roofs that are at the end of their service
life and are deteriorated can rarely have
their service life extended by application of
a roof coating. Problems often arise with the
performance of the applied coating when it
is applied to a very old roof because of inadequate
attachment of the underlying roofing
materials to the roof deck, moisture trapped
within the existing roofing materials, cracks
and gaps in the underlying roofing membrane
that cannot be bridged by the roof
coating, and other issues.
Roofs suitable for coating application
should have the following characteristics:
• The existing
roofing materials must be attached
to the roof structure sufficiently to
meet local code requirements. A
Figure 4 – Coating failure at locations of ponding water.
roof’s uplift resistance is only as
strong as its weakest plane.
roof should first be repaired by
patching the split seam with an
Applying a coating on top of poorly
attached materials will not improve
the insufficient attachment of the •
EPDM membrane patch before any
coating application.
•
underlying roofing.
The roof
substrate should be sound and free
Existing moisture
in the roof system must be removed
as part of any roofcoating project.
from large cracks, split seams, and
gaps. Roof coatings are generally
able to bridge small cracks and gaps
in the underlying membrane and
may be able to fill small pinholes.
However, the coatings generally are
The importance of removing moisture
from existing roofing systems to
be recovered or coated is well understood
by the industry and is discussed
in industry standards. When
evaluating a roof as a candidate for
not intended to repair major defects
in the underlying roofing, and their
ability to bridge cracks is dependent
upon the degree of movement that
the crack experiences. Coatings
should not be expected to bridge all
roof coating application, the designer
must be confident that the existing
moisture in the roof can be
removed. The new roof coating
should be applied to a clean, dry
substrate.
•
cracks.
Major existing leakage
problems must be addressed
before application of the roof coating,
including removal of existing
• Adequate
slope-to-drain is a factor in the success
or failure of roof systems in
general, and it is especially critical
with roof coatings. Roof coatings
tend to deteriorate more severely in
wet roofing materials. As noted
above, roof coatings may be able to
repair small pinholes and cracks in
existing roofing materials, but the
root cause of major leaks should be
addressed before coating applicaareas
of ponding water than in areas
that drain well (Figure 4). If cracks
develop in the coating, water can
pass through the cracks, get under
the coating, and contribute to
delamination (Figure 5). If the pondtion.
For example, a leak due to a
split seam in an EPDM membrane
ing problems are severe enough, the
roof should not be considered a can-
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Identify a Suitable Coating
Track record.
Existing roof substrate.
Slope-to-drain.
Figure 5 – Cracks in roof coating. Water passes through cracks in the coating and
contributes to delamination of the coating from the roof membrane.
ings (that act like a continuous membrane).
When considering which coating to use,
several factors should be evaluated, including
the following:
• Choose a coating system
that has previously been successfully
installed and has performed
well on the same type of roof.
Designers should confirm that the
coating has been successfully installed
on similar roofs by visiting
the sites of other installations, contacting
building owners with similar
installations, and reviewing photographs
of similar projects. Visiting
the sites of other installations generally
provides the best opportunity to
evaluate performance. Reviewing
manufacturers’ published values for
various physical properties of their
roof coatings alone will not guarantee
adequate performance.
• Roof coatings
perform differently on different
didate for a coating, or a coating that substrates, so designers must be
can perform in immersion condi- Once the design team has determined certain to select a coating that is
tions should be selected. that a roof is suitable to receive a coating, intended for the type of roof at hand.
the next task is to choose a specific coating For example, some roofs move more
system for that particular roof. There are a than others in response to temperawide
variety of dif- ture changes and structural loads,
ferent coating so a coating must be selected that
systems on the can accommodate the expected
market that range movement. If a coating is too brittle
from relatively to accommodate the movement in
thin, unreinforced the roof substrate, it may crack and
coatings (that are delaminate (Figure 6).
more like a paint) • As noted above, if
to relatively thick, the roof has severe and pervasive
reinforced coat- ponding, it should not be a candi-
Figure 6 – Cracking and coating delamination.
Brittle coating cannot accommodate movement
in substrate.
Figure 7 – Water-based acrylic coating performing well
on steeply sloped roof, colored green to match historic
patinated copper. Note: Copper roof in lower right,
shown covered with EPDM (for emergency repairs) in
lower left, and then coated in top of photo.
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Physical Properties to Consider When
Selecting a Coating
Elongation and tensile strength.
Water absorption. Glass transition temperature.
date for coating. However, if the roof
has only small, short-term puddles,
relatively good drainage overall, and
the designer wants to pursue a coatment
as a result
of these temperature
swings.
When a roof coat-
Figure 8 – Cracks in roof coating. Water gets
underneath the coating at cracks and
contributes to delamination.
Figure 9 – Cracks in a brittle coating.
ing, then the coating selected should
have good properties in ponded
water (such as low water-absorption).
The options for coatings that
will provide long-term durable performance
under ponded water conditions
are limited, but polyurethane
or silicone coatings generally perform
better than acrylic coatings
under these conditions. Waterbased
acrylic coatings are typically
more susceptible to degradation
under ponded-water conditions than
other coatings and therefore are
most suited for installation on roofs
with good slope-to-drain throughout
the roof (Figure 7).
As noted above, reviewing physical
properties of coatings alone cannot guarantee
coating performance in service; however,
comparing physical properties is a good way
to compare different coatings and evaluate
them relative to one another. Designers
should focus on certain key physical properties
of coatings; these properties include
the following:
•
Roofing systems can experience
temperature cycles of 150ºF or more
(from winter cold to midsummer
heat), and they must be able to withstand
the stresses imposed by moveing
is applied as part of the roof system,
it must be able to move with
the underlying roofing without
cracking (Figure 8). Since most roof
membranes are highly elastic and
exhibit significant elongation capabilities,
roof coatings must match or
exceed this performance. Coatings
with higher elongation and tensile
strength properties will generally
perform better than otherwise similar
coatings with lower elongation
and tensile strength values.
• As is the case
with most roofing materials, water
absorption leads to premature
degradation of roof coatings.
Depending on the components of a
particular coating, water absorption
may lead to chemical breakdown of
the coating. Absorbed water can also
lead to mechanical stresses within
the coating due to freeze-thaw
cycling. Cracks in the coating can
exacerbate the degradation from
freeze-thaw cycling by allowing additional
water into the coating, resulting
in larger cracks and more water
intrusion.
The water absorption characteristics of
a coating are not included on some coating
manufacturers’ product data sheets,
though they can be easily determined by
laboratory testing. Water absorption characteristics
can also change over time, particularly
in a coating exposed to the elements
in service. We tested several waterbased
acrylic coating samples in our laboratory
both before and after approximately
one year of exposure to the weather and
found that the water absorption characteristics
did not remain constant. Additional
research is needed to identify an appropriate
threshold for water absorption in roof
coatings; but in relative terms, lower water
absorption will result in better coating performance.
• The
glass transition temperature (Tg) of
a coating is the temperature above
which the coating remains flexible.
Below the glass transition temperature,
a coating can become brittle
and prone to fracture (Figure 9).
Because a roof coating must be able
to accommodate the movement of
the underlying roofing without fracturing,
it is important that the ambient
temperature where the roof coating
is in service remain well above
the coating’s Tg throughout its service
life. This point becomes especially
important in cold-weather climates
where wintertime temperatures
may approach the Tg of many
coatings. To provide the best performance,
the ambient temperature
should not be lower than a coating’s
Tg.
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The Tg is typically not included among
the physical properties listed on a coating’s
product data sheet. The data can be
obtained through laboratory analysis of
coating samples, though a coating’s manufacturer
may also be able to provide the Tg
for its coatings on a case-by-case basis.
• The adhesion between
the coating and the underlying roofing
is critical to the successful performance
of the roof system.
However, the adhesion is a function
of many factors, including the substrate
condition, substrate preparation
procedures, environmental conditions
during application and
immediately after, and application
methods. Therefore, while strong
adhesion is important and should be
evaluated during application, it is
not exclusively a physical characteristic
of an individual coating.
Once a roof has been identified as a
good candidate for coating application and
an appropriate coating product has been
selected, the following procedures should be
followed to give the coating application the
best chance of success:
• Roof coating specifications
should include requirements
for a thorough in-place mock-up
process. The mock-ups should
include review and approval by the
designer of the substrate preparation
procedures prior to application
of the coating, as well as review and
approval of the coating application
itself. Unique mock-ups should be
performed for each substrate type
and each coating type on the job.
While a review of the coating mockup
for aesthetics should be included,
it should not be the primary
focus. The mock-up should demonstrate
that the coating, as applied to
the specific roof substrate, will meet
all technical performance requirements
set for the project. The mockups
should include testing and
involvement of the manufacturer as
described below. However, a mockup
cannot be used to evaluate the
long-term performance of the coating
system. Hence, it is important to
use coatings with a successful track
record on the same substrate, and
use the mock-up to evaluate constructability,
initial performance,
and aesthetics.
• Designers
typically rely on manufacturers with
a long-term track record of reliable
performance, but if the performance
history is not complete or long
enough, or if there are questions in
the performance history, third-party
testing of the physical properties of
coatings may be appropriate to confirm
that they are meeting the
required performance standards. If
possible, testing should be performed
during the mock-up process
to show that the new coating has the
properties specified. Depending on
the size of the project, it may also be
appropriate to test periodically during
the construction (similar to periodic
weld inspections in a PVC roof);
however, it will take some time to
perform the tests, which may make
it difficult to take corrective action in
time if necessary. Some of the coating
properties that could be tested
include elongation, tensile strength,
water absorption, and adhesion.
Testing the Tg may also be appropriate,
especially in cold climates.
•
Throughout the specification, mockup,
and application process, the
designer and installer should maintain
a close relationship with the
manufacturer of the roof coating
being used. Because of the complex
chemistry involved in making the
coatings, the manufacturer is in the
best position to understand nuances
about how the coating will perform in
certain situations and can make
informed recommendations about
how best to approach a unique application.
Following manufacturers’ recommendations
alone will not ensure
a successful roof coating application,
but manufacturers’ unique knowledge
of their products can prove
especially valuable when applied in
conjunction with mock-ups and
thorough performance testing.
Finally, after a roof and coating have
been selected and all possible steps have
been taken to provide a quality installation,
the last factor in coating application success
is maintenance. All coatings, whether
they are roof coatings or other types,
require periodic maintenance to keep up
performance. Regular maintenance of roofs
and any applied coatings is particularly
important given the harsh environment
where the coatings are installed. Roofs and
their coatings should be closely inspected at
least annually. The inspection should
include walking all areas of the roof and
performing an up-close, hands-on inspection
of any areas exhibiting signs of deterioration
(cracking, staining, peeling, blistering,
etc.). An up-close, hands-on inspection
should also be performed in areas prone to
premature deterioration, such as valleys,
low points where water ponds, high-traffic
areas, and beneath mechanical equipment.
Any areas of deterioration should be
repaired promptly, including repairing any
damage to underlying roofing materials and
replacing any wet roofing materials. At
some point, the coating system (i.e., original
coating with additional maintenance applications)
may become so thick that the coating
material will not perform as intended.
There is a wide range of reasonable
expected service lives for roof coatings due
to the variety of coatings available and the
variety of roofs to which they are applied. A
five-year lifespan is a reasonable expectation
for a roof coating when the preceding
guidelines are followed. At the other end of
the spectrum, some roof coatings may last
15 years or longer with proper maintenance
and ideal roof and environmental conditions.
It is important that the designer,
owner, and manufacturer make an honest
assessment of what is reasonable to expect
before embarking on a coating application.
SUMMARY
Roof coatings can extend the service life
of an existing roofing system and might provide
additional benefits such as improved
aesthetics or energy performance, but these
benefits will only be achieved if the existing
roof is a suitable candidate for coating and
an appropriate coating system is selected.
The roof coating must also be properly
applied and maintained to realize these
benefits.
Adhesion.
Specifying Roof Coating Applications
Mock-ups.
Testing and sampling.
Manufacturer involvement.
Maintenance
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