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The gloss level on your furniture is a major aesthetic quality your customers are looking for — making it important to consistently achieve the desired gloss level.
Do you have consistent gloss on your finished parts? The gloss level rating specified on the container may not be the same as the gloss level rating you achieve on the end product.
Many factors affect the gloss of a finished piece of furniture. Some of the factors that have a noticeable effect on gloss include, of course, the gloss level of the finish as produced, amount of finish applied, type of finish, sanding of the substrate, sanding between coats, porosity and species of wood, and any distressing or rubbing and polishing of the finish.
It is important to know how the gloss level of the finish is determined by the finish manufacturer in order to understand why the applied gloss level can vary.
Many finish manufacturers use specialized equipment to produce consistent results in the laboratory. Common equipment includes a draw-down bar designed to apply a specific wet film thickness consistently. Draw-down bars can apply wet films of less than 1 mil or up to and over 20 mil. The draw-down is usually applied to black glass, which provides a consistent substrate free of porosity variance and a very level surface for consistent application.
The gloss of the finish is then checked with an electronic gloss meter designed specifically to provide a numeric gloss reading. These gloss meters can measure gloss variances much better than the human eye. The gloss meters are produced with different testing angles depending on the range of gloss you need to evaluate.
Check with your finish supplier about its testing process and equipment. If at all possible, obtain a gloss meter designed to test the gloss range you desire. Without a gloss meter it is difficult to produce verifiable results. A small range (usually ±2 degrees) of acceptable gloss is usually specified. It is difficult for the human eye to tell gloss variances of less than 5 degrees. Three wet mil is a common thickness of draw-down to use for gloss evaluations of clear finishes. As you can tell, this process, while very consistent, is different than spraying multiple coats of finish on a piece of wood.
Gloss levels
Now that we know each finish process is different than the actual gloss level test in the laboratory, we can look at the factors in your factory that will affect the gloss level you actually achieve.
The first is the amount of finish applied. How many coats are you applying and how many wet mil are you applying with each coat. Usually gloss levels increase with increased amounts of finish applied. It is vital to apply consistent amounts of finish to achieve consistent results.
A simple test to run in your factory is to use a wet film thickness gauge when spraying each coat of finish. The total wet film thickness should add up to the same amount from piece to piece. When additional coats of finish are applied to only some parts, it is usually noticeable when they are placed side by side. Check with your finish supplier or equipment supplier to obtain wet film thickness gauges and learn their proper use.
The type of finish (i.e., lacquer, catalyzed varnish, catalyzed lacquer, two-component polyurethane, etc.) will also affect the final gloss you achieve. While the thickness of the finish can affect the gloss, it is especially true for certain types of finish. Lacquer is usually affected more than a catalyzed varnish. This is largely due to the lacquer's ability to rewet the previous coat and mix together; usually a noticeable increase in gloss is seen with each coat. Check your finish with your finishing process to see what gloss variances show up and check with your finish supplier for more information about getting a consistent gloss level.
Sanding can also affect the final gloss. Any sand marks that the finish cannot fill or level out will cause a distortion of the finish surface, which will scatter light and lower the gloss level. It is important to make sure you are using the proper grit sandpaper for the sanding application.
Highly porous woods such as poplar and alder will cause gloss reductions until the pores of the wood have been filled. When the pores are not completely filled, the resulting distortion on the surface of the finish will scatter light, thereby lowering the gloss level. Also, open-grained wood that is not completely filled, such as oak and ash, will give a perceived lower gloss level in the grain area vs. the rest of the piece. When checking open-grained woods with a gloss meter, it is important to line up the gloss meter along the hard wood to get an accurate gloss measurement and prevent an erroneous reading over an open grain area as the gloss meter does not realize where the grain starts and stops like the human eye does.
Finally, any distressing or rubbing will alter the gloss. If you are rubbing the surface, it is important to check the gloss part-to-part to make sure you are rubbing the finish to the same gloss level consistently.
Ed. note: David Jackson has held various titles including laboratory technician, formulator, senior chemist and general manager for two wood coatings companies. Jackson currently is the general manager for Jasper Chemical Coatings in Jasper, Ind.
author: By David Jackson