ASTM D7787/D7787M-13(2022)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D7787/D7787M − 13 (Reapproved 2022)
Standard Practice for
Selecting Wood Substrates for Weathering Evaluations of
Architectural Coatings
This standard is issued under the fixed designation D7787/D7787M; the number immediately following the designation indicates the
year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last
reapproval. A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This practice offers guidelines for selecting wood or 2.1 ASTM Standards:
wood composite substrates for the evaluation of specific D4214 Test Methods for Evaluating the Degree of Chalking
weathering performance characteristics of architectural coat- of Exterior Paint Films
ings such as exterior paints, primers, and deck finishes. G147 Practice for Conditioning and Handling of Nonmetal-
lic Materials for Natural and Artificial Weathering Tests
1.2 Proceduresincludeselectingwoodspeciesandchoosing
individual wood test panels through visual examination of the
3. Terminology
wood characteristics.
3.1 Definitions:
1.3 This practice is intended to cover the most commonly
3.1.1 weathering performance characteristic, n—an attri-
employed wood substrates used in weathering studies of
butedisplayedbyacoatingafteraperiodofexposuretonatural
architectural coatings. It is not intended to serve as a compre-
sunlight, UV light, moisture, varying temperatures or other
hensive guide for all wood species that may be employed for
environmental elements.
the purpose of evaluating weathering performance character-
3.1.1.1 Discussion—Weathering performance characteris-
istics of architectural coatings.
tics may include film integrity attributes such as checking,
1.4 The values stated in either SI units or inch-pound units cracking, peeling, flaking, erosion, chalking; appearance attri-
are to be regarded separately as standard. The values stated in butes such as gloss or color retention; or resistance to dirt
each system are not necessarily exact equivalents; therefore, to pickup, mildew, or algal defacement.
ensure conformance with the standard, each system shall be
3.1.2 wood condition factor, n—an attribute of a wood panel
used independently of the other, and values from the two
that indicates its state prior to being used in a coatings
systems shall not be combined.
performance evaluation.
1.5 This standard does not purport to address all of the 3.1.2.1 Discussion—Wood condition factors may include
safety concerns, if any, associated with its use. It is the duration of time since the panel was milled, chemical pre-
responsibility of the user of this standard to establish appro- treatments, or exposure to environmental elements prior to
priate safety, health, and environmental practices and deter- being coated.
mine the applicability of regulatory limitations prior to use.
3.1.3 wood panel, n—a thin, flat substrate material com-
1.6 This international standard was developed in accor-
posed of any species of wood or wood composite.
dance with internationally recognized principles on standard-
4. Summary of Practice
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
4.1 The experimenter first determines the critical weather-
mendations issued by the World Trade Organization Technical
ing performance characteristics of the architectural coating
Barriers to Trade (TBT) Committee.
being tested and determines the species and lumber grade of a
wood or wood composite that is similar to the in-service
application of the wood that is to be studied.
This practice is under the jurisdiction of ASTM Committee D01 on Paint and
Related Coatings, Materials, and Applications and is the direct responsibility of
Subcommittee D01.42 on Architectural Coatings. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Jan. 1, 2022. Published January 2022. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 2013. Last previous edition approved in 2013 as D7787/D7787M – 13. Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/D7787_D7787M-13R22. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7787/D7787M − 13 (2022)
4.2 Wood species are selected based on the weathering 6.2 Select wood characteristics for the experiment that will
performance characteristics of interest and the species that will provide the most relevant results for the exposure experiment
likely be used for the studied application. in the desired timeframe. Wood features and wood condition
factors (as defined in 3.1.2) together with coating formulation
4.3 Individual test panels are selected based on visual
and application method, including dry film thickness or cov-
observation of the following features: (a) growth patterns (rate
erage spread rate will determine the speed and extent of
of growth, % latewood, grain, etc.), (b) grain orientation, (c)
exhibited weathering performance characteristics.
characteristic feature (knots, checks, glue lines, etc.), and (d)
6.2.1 Pre-weathering of wood substrates affects the weath-
color characteristics frequently associated with heartwood or
ering performance of coatings. Significant changes in the
sapwood of the wood and the type of wood extractives of the
weatheringresultsofcoatingsmaybeobservedfromperiodsof
in service wood. Selection should be done using quantitative or
pre-weathering as short as one week. For this reason, adequate
qualitative criteria to ensure the best substrate representation
pre-weathering of panels is required if paint is to be applied
for the test series. When evaluating multiple coating samples
sometime after wood installation on the structure.
on the same board, in selecting the board, care must be taken
6.2.2 Different types of wood chemical or thermal
to assure that the areas designated for each paint sample
treatments, composite glue lines or type of binder used may
contain similar characteristic features mentioned above to
impact a coatings performance or longevity.
insure proper comparison of coating durability.
6.3 Select Wood Species (younger than 60 years old):
6.3.1 Southern Yellow Pine (Pinus spp.)—This wood type is
5. Significance and Use
among the most commonly employed wood substrates for
5.1 Natural weathering tests can take several years and
weathering tests due to its availability and low dimensional
accelerated weathering evaluations often run for cycles requir-
stability. It includes species such as shortleaf, slash, loblolly,
ing several weeks or months before obtaining useful data.
and longleaf. Pine is often selected where film deterioration
Correlating wood panels selection with desired weathering
characteristics such as cracking, checking, and flaking are
performance characteristics assists a coatings experimenter in
required. Pine boards tend to flex and expand more than other
maximizing information in the desired time frame.
wood species in varying environmental conditions. Specific
5.2 Because of the long time spans required for exposure
features of pine species are resin pockets and bleeding resin.
testing, it is critical to plan a reliable experiment while
For this reason, pine lumber used in a majority of architectural
controlling as many variables as possible. For this reason,
applications such as siding or trim boards must be kiln dried at
selection of experimental panels made from representative
a high temperature to set the resin.
wood species or composites, grades of lumber and surface
6.3.2 Cedar—Theterm“Cedar”isageneraltermthatcanbe
types that are expected to be painted is very important.
applied for True Cedar (Cedrus spp.) but is also used for
Western Redcedar (Thuja plicata) and Eastern Redcedar (Ju-
5.3 This practice may be used for any types of exterior
niperus virginiana). Cedars generally will give a more dimen-
architectural coating intended for use on wood or wood
sionally sound surface than Pine species. Film durability
composite substrate such as exterior house paints, primers,
failures will normally take longer over cedar than pine. Cedar
wood stains, or waterproofers.
panels can be used to assess gloss and color retention, and it
can be used to observe tannin stain-blocking resistance.
6. Procedure
6.3.3 Redwood (Sequoia sempervirens)—Generally selected
6.1 Determine Critical Weathering Performance Character-
for its heavy tannin content to observe stainblocking effective-
istics:
ness of architectural primers or topcoats.
6.1.1 The determination of critical weathering performance
6.3.4 Pressure Treated Pine—Includes various chemically
characteristics is at the discretion of the experimenter depend-
treated wood. Pressure treated lumber is generally used for
ing on the performance indicators of interest for the coating
studying weathering effects of deck finishes, wood stains and
system being evaluated.
waterproofing coatings. Pressure-treated pine is often selected
6.1.2 Weathering characteristics typically of interest may
to study erosion or film deterioration of waterproofing
include:
coatings, and long-term water repellent characteristics.
6.1.2.1 Film deterioration factors such as cracking, peeling,
6.3.5 Engineered Wood Trim (EWT) and Siding—Includes
flaking, chalking, or blistering. manufactured wood products composed of wood segments,
6.1.2.2 Ability of coating to adhere to wood or wood flakes, particles, or fiber-based composites, or combinations
thereof. Use of EWT and siding now command a significant
composite surface over extended periods as the substrate
expands and contracts due to moisture, or thermal influences, share of the wood cladding market. Thus, it is critical to
evaluate these products in any general architectural finish
or both.
performance evaluation.
6.1.2.3 Visual appearance factors such as color or gloss
retention.
6.4 Selection of Wood Test Panels for Natural Weathering:
6.1.2.4 Extractive bleeding or other paint discoloration as-
NOTE 1—The techniques described for selection of wood test panels
sociated with the wood substrate.
may be applied to any wood species and are not limited to the species
6.1.2.5 Resistance to fungal or algal defacement.
listed previously.
6.1.2.6 Resistance to dirt pick-up. NOTE 2—Supplementary information related to wood structure and
D7787/D7787M − 13 (2022)
features affecting paint performance can be found in relevant literature,
6.4.5 Wood Surface Finish—Depending on the sawing and
including publications.
finishing equipment used, and the desired architectural
6.4.1 Panel Thickness—Although the exact size of the wood appearance, the wood surface could be finished from very
test panel is not critical, natural weathering exposures are smooth to rough sawn. There is a known difference in paint
typically performed on boards 0.9 to 1.2 m [3 to 4 ft] in length, durability on smooth versus rough wood with paint usually
13 to 20 cm [5 to 8 in.] wide, and 0.6 to 2.5 cm [ ⁄4 to 1 in.] in lastinglongerontheroughsawnwoodsurface.Forthisreason,
thickness. Boards with thickness greater than or smaller than panels used for testing should have the surf
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