Measuring accurately matters in woodworking and finishing. There are many things that need to be measured accurately for optimal finishing. Last month’s Why Finish Measuring Accuracy Matters part 4 covered measuring dry film thickness. This month gloss is covered.
Finishes provide a specific gloss at the surface of the finished part. Obtaining the specific gloss can increase the perceived value of the wood being finished. High gloss is commonly used on conference tables and other office furniture. When polished to a “wet look” the high gloss can impart a very high end formal appearance to the furniture. Commonly satin and flat finishes are used to provide a more relaxed feeling while maintaining some sophistication. Now dead flat and even lower gloss finishes are providing a “natural” look where it is difficult to tell the wood is finished. Some of these extremely low gloss finishes also protect the wood as good as or better than some traditional finishes.
Gloss level is defined as the amount of light that reflects in a specific direction. The higher the gloss level the more light is reflected in the same direction as in a mirror. Low gloss levels mean the light is dispersed in a variety of directions.
Gloss units for wood finishes cover a range of 0-100 GU, with 0 being the lowest and 100 the highest. The human eye can usually distinguish between gloss differences of 5 or more gloss units (GU) side by side. That is for most of the gloss range, however at the lowest gloss levels we can see a difference of just a couple of gloss units. Because we can distinguish between many levels of gloss there are usually a few to several choices of different gloss levels within a coating product line.
Measuring accurately requires equipment to measure the gloss level. Gloss meters have improved significantly over the last couple of decades and can easily measure well beyond what the human eye can see. Often gloss meters can measure to a tenth of a gloss unit and can vary in price between a few hundred dollars to a few thousand dollars.
Many things can affect the final gloss of a finished part that need to be controlled. Lower sheen coatings use flattening agents to lower the sheen to a specific level. These flattening agents tend to settle, sometimes a lot. They must be fully mixed and stay mixed to provide a consistent gloss level. Second, some finishes increase noticeably in gloss level at a higher film thickness. Therefore it is important to apply the same amount of film thickness with each coat and use the same number of coats for every part. Third, finish problems can affect the gloss as well. These problems include; blush, entrapped air, orange peel, dry spray, etcetera. All of these problems need to be prevented to achieve consistent results.
Gloss is not gloss is not gloss. It may seem that gloss measurement would be simple and consistent across all industries and equipment manufacturers, however it can be complicated. The angle the gloss meter measures the gloss at makes a tremendous difference in results achieved on the same surface. Gloss meters often measure at 20o, 60o, or 85o and some can measure at all three angles. The most common angle used for wood finishes is 60o. Also, the standards the gloss meter is calibrated to can vary by meter manufacturer. Sometimes a correlation between gloss meters has to be determined to compare values especially when the finisher uses a different gloss meter than the finish manufacturer. Always consult with your finish manufacturer on the best methods to measure the gloss of the finishes you are using.