Performance Testing of Exterior Wood Coatings

Wood is one of the most utilized building materials for exterior structures. Like most exterior substrates, wood structures need protection from the environment. While some wood types are naturally resistant to the environment (e.g., cedar) and some wood is treated with chemicals to preserve its integrity (wolmanized timber), paints and coatings are used to both protect wood and enhance its appearance. Without protection, the wood would gradually erode. A properly coated wood structure should last approximately 8 to 10 years before there is a need for repainting; however this is dependent on the degree of surface preparation, the quality of the paint, consistency of the application, orientation of the surfaces (horizontal versus vertical) the location (prevailing temperature, humidity and intensity of solar radiation), and other factors. An important step to avoid premature failure of the protective coating is by testing candidate coatings using various laboratory performance tests. The selection of performance tests is dependent on what type of environmental factors the coating will be subjected to. In addition, it is important to know the existing condition of the wood substrate (e.g., new bare, pre-primed, or weathered). These details will help predict the types of problems that may occur with the exterior wood coating.

While all substrates expand and contract to a degree, one challenge pertaining to protection of wood is that it swells and shrinks a lot. The fibrous nature of wood makes it susceptible to moisture absorption and swelling. This can occur with most exterior wooden structures even if the swelling is minimal since the wood naturally expands and contracts in response to temperature and humidity changes. The coating applied to the exterior wood must possess good flexibility properties to avoid cracking during this physical change of the substrate. An effective way to test the flexibility of a coating is ASTM D522, “Standard Test Methods for Mandrel Bend Test of Attached Organic Coatings.” This method includes two procedures to determine the coating’s resistance to cracking. The first method entails bending coated panels over a conical (horizontal cone) mandrel. If there is cracking observed, the length of cracking is measured, and the percent elongation is determined using a plotted curve that represents the relationship between percent elongation and the diameter of the conical mandrel. The second method entails bending the coated panels over cylindrical mandrels of various diameters. This method can be used as a “pass/fail” criteria (whether cracking is observed or not at a specific mandrel size). The choice of mandrel size is typically associated with the coating type being tested or the structure shape and size. In addition, the resistance to cracking can be determined by bending the panels over the mandrels, starting with a larger diameter mandrel and repeating the process using small diameter mandrels, until the coating cracks.

wood coatings

Another laboratory test method to evaluate a coating’s flexibility or elongation is by performing ASTM D2370, “Standard Test Method for Tensile Properties of Organic Coatings.” This method uses a tensile testing apparatus to determine the elongation, tensile strength, and stiffness of a coating when tested as a free film. The free film of coating is clamped at the top and bottom ends to grips at an agreed upon gage length. The film is then pulled at an agreed upon rate of elongation until there is a rupture of the film. The specimen elongation can be determined using the increase in jaw separation data. The tensile strength of the material can be determine using the force required to rupture the film. Additionally, the stiffness, or modulus of elasticity of the material can be calculated from the initial slope of the elongation versus load curve.

Horizontal surfaces of coated wood structures such as decks can suffer wear from foot traffic, pressure and movement of furniture, and weather. Foot traffic and furniture dragging can lead to abrasion of the coating material or weaken the adhesion of the coating to the substrate. One way to test the abrasion resistance of the coating is by performing ASTM D4060, “Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser.” Briefly, this method involves mounting a 4” x 4” coated panel to a turntable. The panel is rotated on the turntable beneath two weighted abrasion wheels. The abrasion resistance of the coating is calculated as the loss in weight of the coated panel after the number of required cycles is completed under a specific load and type of abrasion wheel (e.g., 1000 cycles using CS-17 wheels under a 1000-gram load). Alternatively, this method can reveal the number of cycles required to remove a certain amount of coating by conducting pre- and post-coating thickness measurements.

One limitation to these three methods for evaluating wood coatings is that they cannot be performed on a wood substrate, so the flexibility and wear resistance of the coating is evaluated independent of the substrate effects. None-the-less, the testing can be used to evaluate relative performance.

Numerous factors on a coated wood structure can lead to loss of the coating adhesion; therefore, adhesion is an important property to evaluate for wood coatings. The wood substrate type, e.g. primed, weathered, or pre-treated, will likely have a considerable impact on the adhesion properties of the coating to the structure. Oftentimes coating applied to new wood that is properly pre-treated or primed exhibits better adhesion properties than coating applied to weathered wood or an older wood substrate that may have a previously applied coating on the surface. Older weathered wood is frequently protected using coatings, so the adhesion of the applied coating to this condition is important to test. Arguably the best method to test the coating’s adhesion to a specific type and condition of substrate is by performing ASTM D3359, “Standard Test Methods for Rating Adhesion by Tape Test.” Whether you are testing a coated wooden structure in the field, or a prepared wooden panel in the lab, the methods involve making either an X-cut (Method A for coatings > 5 mils thick) or a cross-cut (Method B for coatings < 5 mils thick)  through the coating film to the wood substrate. Pressure sensitive tape is applied over the incisions, removed, and the amount of coating detachment is rated on a scale of 5 to 0. The test is destructive, so the number of tests should be limited if performed on the actual structure, and the test areas may need to be repaired.

Scrape adhesion is another way to test the adhesion of coating to a wooden substrate such as a deck that will be subjected to furniture dragging across the surface. ASTM D2197, “Standard Test Method for Adhesion of Organic Coatings by Scrape Adhesion” can be performed using coated flat panels. The resistance to scrape adhesion loss is evaluated by pushing the panels beneath a rounded stylus that is continuously loaded with increasing weights until the coating is removed from the substrate. The load weight at the adhesion failure end point is reported. Note that this test can only be performed in a laboratory on panels.

Coated wooden structures can experience premature coating failure before ever being put into service. Pre-coated wooden panels at times are stacked face to face or face to back during warehousing, packaging, and shipment. These panels can be stacked for long periods of time and be exposed to higher pressures, heat, and moisture depending on where the panels are shipped to and the storage conditions. It is important for wood coatings that will be subjected to these conditions to have good block resistance, which is the ability of a coating to resist sticking to another surface. Additionally, a coating with good block resistance will resist a change in appearance when it is pressed against a surface for an extended period. One of the best ways to test the block resistance of a coating on a wooden substrate is by performing ASTM D2793, “Standard Test Method for Block Resistance of Organic Coatings on Wood Panel Substrates.” Briefly, this test method involves stacking coated wooden panels in a manner such that both the coated faces are touching as well as a coated face to the back side of another panel. Typically, this stack involves six coated samples. The stack is subjected to an agreed upon pressure and temperature for a specific amount of time. The pressure is then released, and the coated surfaces of the panels are observed.  The surfaces are rated by their degree of blocking from no surface damage to greater than 50% surface damage. If no sticking or damage to the coating film is observed, then the material is considered satisfactory. Similar to scrape adhesion, block resistance can only be performed in a laboratory.

An exterior coated wood structure subjected to frequent moisture and warm temperatures may be susceptible to fungal growth. Fungus on the surface of a coated wood structure can cause staining, coating degradation, and can be a health concern. A coating that is intended to be applied to an exterior wooden structure in this environment should be subjected to fungal resistance testing prior to the application. ASTM D3273, “Standard Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber” can be used for both exterior and interior coatings to test for resistance to accelerated mildew growth. The test method employs an environmental chamber that maintains high temperature and humidity conditions (approximately 90°F and 95% relative humidity). Within the chamber the coated wooden panels are suspended above inoculated soil with enough space for air circulation. The coatings on the panels are rated weekly for mold growth using a 0 to 10 scale by estimating the amount of surface defacement, with 10 being no defacement and 0 being complete defacement. The method notes that performance at a certain rating does not imply a specific time period for a fungal free coating, however a higher rated coating will typically perform better in the actual environment.

wood coatings

Finally, one of the best ways to evaluate the durability of an exterior coating on wood is by subjecting it to accelerated weathering in a laboratory. There are several accelerated weathering test methods, but a good one to use when testing coatings on wood is ASTM D4587, “Standard Practice for Fluorescent UV-Condensation Exposures of Paint and Related Coatings.” This accelerated exposure can be used to illustrate the cumulative effects of sunlight, moisture, and heat on a coating applied to wood. Briefly the coated wood panels are conditioned for a minimum of 7-days then placed in a fluorescent UV/heat and condensation chamber. The method suggests using Cycle 3 for exterior wood coatings, which consists of 4 hours heat/UV at 60°C followed by 20 hours condensation at 50°C. After an agreed upon exposure time (e.g., cumulative 500 hours), the coated panels are observed for blistering, cracking, adhesion loss, color change (if pigmented), and/or gloss reduction. Since actual exposure environments cannot be replicated in the laboratory, there is no correlation between exposure time and years of performance. However, this regime works well for comparative testing.

Outdoor exposure testing of candidate coatings on representative wood substrates in a variety of natural environments is ultimately the best way to assess the performance of the coating; however, it may take years before discerning performance is evident. In that time, raw materials and formulations may change making any data obsolete.

In conclusion, there are many man-made and natural environments that can affect the performance of coatings applied to wood structures. This article described some of the laboratory test methods that can be performed on candidate coatings to assess comparative performance. When selecting a test method, it is important to predict, to the extent feasible what type of environment the coating will be subjected to and what type and condition of wood substrate it will be applied to. Each coating project is unique, and these test methods may assist in selecting the right coating system for an exterior wood structure.

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