A Comparison of Method of Evaluating Exterior Natural Finishes

The purpose of this study was to investigate six quantitative techniques in order to obtain a test method that would yield measurable data to agree with a visual durability rating after exposure in a weatherometer. Panels of dark red Philippine mahogany were finished with 18 proprietary finishes. After exposure in a weatherometer for 865 hours, relative durability was determined by visual inspection and the finishes were divided into four classifications according to their durability. The measurable data were then plotted against the time of exposure. To obtain basic information regarding the physical characteristics for each of the products tested, determinations were made of viscosity, density, and percent solids content. By means of photoelectric tri-stimulus data, quantitative expressions were obtained to correspond to the perceived, amounts of surface color changes. There seemed to be a correlation between the density of the finish and durability. Those finishes with a density of 0.89 grams per cubic centimeter or larger were better in durability than those below this density figure. There was very little variation between panels from the same or different boards. Four visual classifications were used to rate the finishes for durability: good, fair, poor, and, failure. Although some of the graphical representations were able to separate the “good” from the “failure” group, there was usually at least one finish that was not separated. The finishes in the “fair” and “poor” classifications were seldom separated to any extent. Only in the analysis of color was there a sufficient spread between the various finishes to be clearly observable.

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