Paint products, such as varnishes and lacquers, that exhibit low viscosity use efflux cups such as Ford cups (ASTM D 1200) or Zahn cups (ASTM D 4212). The higher the KU number, the more viscous the paint. This unit of viscosity is expressed in Krebs units, or KUs. The resistance created by the paint is determined by varying the weight attached to the paddle until the paddle makes 100 revolutions in a period of 27-33 seconds.

The Stormer viscometer uses a small paddle that rotates through the paint. The most commonly used method determines low shear viscosity of paint using a Stormer viscometer following procedures outlined in ASTM D 562. Each is used to measure a different combination of rheological properties in paints This section will address the four most commonly used methods. Viscosity can be determined using a variety of methods. Fineness of dispersion is determined following procedures outlined in ASTM D 1210. Higher readings indicate finer dispersion, which should be expected with semi-gloss and gloss paints. The depth at which continuous groupings of particles or agglomerates protrude through the surface of the wet film is the fineness of grind. The most commonly used unit of measure is the Hegman unit, which varies from a rating of 0 (4 mils) to 8 (0 mils) in increments of 0.25 mils. One method for measuring the degree of dispersion (or “fineness of grind”) is to draw the liquid coating down on a grind gauge, which has a calibrated tapered groove varying in depth typically from 4 to 0 mils. Proper dispersion maximizes color strength, uniformity, gloss control and other properties of the finished paint. Generally, the more finely a pigment is dispersed, the more efficiently it is used. In this edition, we will discuss how to determine the fineness of grind and viscosity of a wet paint sample, and various aspects on appearance of the dry paint, including gloss, color and hiding.

How to test paint viscosity