Seven seconds into Midwest Prefinishing’s video tour, a stainless steel viscosity cup comes briefly into view. You’ll miss it if you’re not careful; the snappy music and images move rapidly. It symbolizes the ever-vigilant, behind-the-scenes, adjustment necessary for a quality finish as temperature and seasons change.
Basic Q&A on Viscosity
Because we’re moving from spring into summer, it’s timely to review some basic information about viscosity. If you dig too deep, you’ll need a math degree. You may be up to it; but we’d prefer to skip the math.
- What is viscosity?
Viscosity measures a finish’s resistance to flow, its thickness. The correct flow improves finish application and uniformity. Plus, it controls product cost.
- What makes it change?
Temperature, humidity, and atmospheric pressure affect viscosity. When temperature falls, viscosity rises; when temperature rises, viscosity drops.
“Most materials change in viscosity as a function of temperature. Those normally measured with viscosity cups change in the range of 3-8% per degree Celsius change…” (www.gardco.com)
- How is it measured?
Dip the viscosity cup into the material. As you lift it out, begin to time how long it takes until you see the first break in the stream. Refer to a conversion table in order to find the viscosity. For example, if your test totaled 39 seconds using a Shell #2 cup, the viscosity level is 20 centipoise. Since we’re not doing math, we’ll ignore the origin of the term, centipoise, and its companion terms, poise, stoke, and centistoke.
- Which cup is used?
Although there’s over 20 cups commonly used, it doesn’t matter which one is used. A conversion chart will find a common standard of measurement, the centipoise, regardless of the cup.
- Where is the measurement applied?
The material vendor and/or spray equipment manufacturers recommend viscosity levels for the best performance. If your score doesn’t match, adjust the material accordingly.
Finishing requires a multi-tasker
While monitoring viscosity, other elements demand the finisher’s attention.
“The effects of the weather on the atomization and drying of the finish, the size of the fluid tip, how far away from the surface you are holding your gun, the overall cleanliness of your spray gun, and the shelf life of the product.” (www.highlandwoodworking.com)
Tweaking the right elements within this multi-tasking challenge, requires experience. Mark Larson, Midwest Prefinishing’s owner, spent the last 23 years perfecting his finishing system. And the viscosity cup is always nearby.