Ask the Expert Question-and-Answer Archive

by Ron Joseph

February, 2006

Measuring Paint Viscosity

Q. What are some ways to make sure paint to be used is correct vicosity with gravity fed HVLP sprayer? I do not have a viscosity measure.

A. You cannot guess or eyeball the viscosity of paint. Other than to purchase a viscosity cup, I don't have a method for you. If you are a do-it-yourself painter, then purchase an inexpensive plastic viscosity cup from a paint supply store. If you are more serious, purchase a calibrated stainless steel Zahn cup from companies such as BYK-Gardner or Gardco, both of which are on the Internet. Zahn cups have different size orifices. For very thin low viscosity paints you would use a Zahn #1 cup. For regular solventborne paints a Zahn #2 cup is the most universal, and for waterborne paints and especially latexes, Zahn #4 and Zahn #5 are suitable. However, gravity fed HVLP guns are not suitable for thick, high viscosity paints. Therefore, if you have a need to use a Zahn #4, #5 and perhaps even a #3 cup, you will probably not be able to get the paint to atomize using a gravity fed gun.

More serious professionals, especially those in automotive assembly plants and those who use paint robots use a Ford #4 cup because of its accuracy and repeatability. Paint manufacturers use the Ford #4 cup as well as shear type viscometers, because they understand that paints don't behave as Newtonian fluids, and the shear type viscometers are built to take this into account.

Best wishes,

Ron Joseph