Spray finishing: Understanding & measuring viscosity
By Jim Larin

Photo By Fuji Spray

When spray finishing, viscosity measurements will help you understand the size of needle and nozzle required to effectively atomize your finish. By recording these measurements, you can duplicate your material “recipe” for future projects to ensure consistency in production.

To save you from dusting off your high school chemistry book, listed below is some information about viscosity in relation to spray finishing:

What is viscosity?

Viscosity is the measurement of a fluid’s resistance to flow. Typically, viscosity is measured in
centipoise which is one hundredth of a poise.

What do I need to measure viscosity?

To measure the viscosity of your finish, you will require (a) your desired finish, (b) a viscosity cup and (c) a stopwatch. The most common viscosity cup sizes are the #4 Ford or the #2 Zahn.

How do I measure viscosity?

Viscosity is measured by submerging your viscosity cup into your finish then removing it to see the
amount of time your finish takes to pass through the orifice at the bottom of the viscosity cup. Start your timer the moment you begin lifting the cup out of your finish. Once the cup drains completely or you notice a break in the stream, stop your timer. The amount of time elapsed is called your run- out time.

What does it all mean?

Most manufacturers of spray finishing equipment will include a viscosity index in their manual for you to compare your run-out time with the recommended run-out time as per the coating or equipment manufacturer. By referencing this index, you will be able to confidently choose the needle and nozzle size best suited for spraying your finish.

A basic understanding of viscosity will help any spray finisher improve the quality of their work while removing the hassle and uncertainty of choosing which needle and nozzle size to install on their spray gun.

Source: Fuji Spray. For information call 800-650-0930 or visit FujiSpray.com.


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