Gloss is a determination of the amount of direct light that reflects to the viewer. A mirror reflects light directly towards a viewer without scattering it. The more light that is directly reflected the higher the gloss, the less light that is directly reflected the lower the gloss.
The gloss, also referred to as sheen, of a coating can be critical to the appearance of the final woodwork. It is also important that the gloss or sheen is consistent from area to area and piece to piece.
Determining the gloss of a surface usually requires a capital investment. While standard panels with the finishes applied as during production can be used as a comparison, it is difficult to tell if the standard panel remains consistent in its level of gloss over time. Handling the standard panel can change its level of gloss. This can happen do to wear and from oily or dirty hands. Also, coatings can continue curing, causing a slightly lower gloss over time.
Multiple equipment manufacturers produce gloss meters. These units usually cost from one thousand to several thousands of dollars depending on the specific type of substrates the meters will be testing. Most wood finishers only need a gloss meter with capabilities in the lower half of the cost range. The gloss meter should be able to be calibrated with each use to a standard “gloss” tile. This will allow repeatability from day to day.
Coatings companies tend to check gloss on standardized substrate such as black tile, black glass, or drawdown cards. The coatings are usually applied to the substrate with a draw down bar that can apply the same amount of coatings each time. The standardized substrate and application tool can provide a more consistent process than spraying wood. Sometimes wood is used, but the natural variations in wood and spray application make it difficult to obtain results as consistent as a drawdown.
Gloss meters are great at providing quantitative results. However, there are some important things to watch for to obtain accurate readings.
First calibrate the meter with the standard “gloss” tile. Make sure the tile is properly cleaned to prevent dust and contaminations from providing an erroneous reading.
Test the gloss of a part in a variety of places that the gloss meter can sit flush against. Note that the gloss meter cannot tell where it is. Testing gloss on red oak may provide erroneous readings if the substrate is tested in an open grain area. While the finish may be a certain gloss the irregularities of the grain will scatter light at a greater degree than a smooth substrate.
Over spray or dry spray will result in a lower gloss reading due to the rough surface that will scatter the light more. This means the application process produced the change in gloss instead of the actual gloss of the coating. Other finishing defects can cause similar problems.
The gloss, of some finishes, tends to rise as either more coats of finish are applied or a heavier coat of finish is applied.
Very thin coats of finish may not smooth out the small irregularities of the wood surface and may result in a lower gloss reading.
Coatings that are not “full” gloss usually have flattening agents in them. These flattening agents are irregular in shape and will slightly protrude through the surface of the dried film causing light to scatter, thereby producing a lower gloss or sheen. The flattening agents tend to settle to the bottom of the liquid finish over time. It is important that the finish is stirred well before use and during prolonged use to maintain a consistent gloss.
Consult with your coating supplier from more information on checking gloss and ensuring you are achieving the consistency you need.