February 20, 2022

What is viscosity and why is it important for a great finish

When spraying various metal and wood materials, it’s important to know what viscosity is and how it affects your finishing projects. By better understanding viscosity, your spray gun set-up will see a uniform quality finish, and your future applications become more consistent and repeatable.

What is viscosity?

Simply put, the viscosity of a coating material indicates its ability to resist flow. That thickness of the coating material, in other words, is due to internal friction. Coating materials with higher thickness require greater air atomizing pressure or pump pressure, like when using AAA or Airless equipment, for example.

To obtain a uniform finishing using highly viscous fluids depends on the spray system set-up. Knowing the viscosity measurements and your spray gun’s abilities are important to get a great finish.

Tip: Always obtain Product Data Sheets for each coating. These data sheets are helpful for establishing the initial coating spray viscosity, gun set-up, and pressure settings for a quality finish. So … take time to read those data sheets!

Viscosity measurements

Centipoise is a worldwide measurement of viscosity.

Unfortunately, the spray coating world does not use a common viscosity tool like a standard paint viscosity cup to set the factory/field viscosity. There must be 12 or more viscosity devices worldwide. The most common in the U.S. is Zahn, Ford, and DIN cups.

Each viscosity cup manufacturer makes cups with different capacities and opening sizes for light, medium, and thick coatings. So, measuring uniformity can be a challenge. Viscosity cup conversion charts are the best references to use to figure out the right spray viscosity across different cup brands.

Recommended spray viscosity

It helps to know what spray gun you have first. Then, we can recommend the spray viscosity. For example, for a Zahn, Ford, or DIN cup, we can advise which spray gun opening/nozzle size and atomizing air pressure will likely be your starting point. Refer to the conversion chart for a recommended viscosity.

You can cross-reference viscosities in seconds between four different viscosity cups at a given Centipoises.

For reference, Viscosity Conversions for Newtonian Fluids:

25ºC (degrees Celsius) is equal to 77ºF (degrees Fahrenheit).

CentipoisesFord Cup #4Zahn #2Zahn #3Zahn #4DIN 4Krebs Stormer
1.09
10.01612
15.01715
22.013.61917
32.015.32020
50.019.02223
65.022.02728
85.027.03430
100.030.0411232
125.036.049141139
140.040.05816134652
165.046.06618145054
200.050.08223175556
225.055.025186459
250.068.027206761
275.074.032227062
300.081.034247463
320.086.036257964
340.091.039268665
370.099.041289466
400.0107.0463010067
435.0116.0503311068
470.0125.0523412269
500.0133.0573713471
550.0146.0634078
630.0167.0684485
885.0199.06495
1070.0270.0100
1290.0105
1760.0114
2270.0129
2700.0136
3620.0
4630.0
6340.0
9850.0
14800.0

Most coatings are formulated to be sprayed at 73ºF to 77ºF (21ºC to 25ºC). Check the viscosity and spray the coating in this temperature range. Every effort should be made to reach these temperatures for a uniform quality finish.

How to test viscosity

What you need:

  • Viscosity Cup
  • Stopwatch
  • Thermometer with coating temperature conversion chart

Steps:
Prepare the material to be tested. Activate, reduce, mix, strain, etc.

  1. Raise or lower coating temperature to 77º F.
  2. Submerge or fill the viscosity cup with coating.
  3. Raise the cup out of coating or open flow from the opening and start the stopwatch.
  4. Always keep your eye on the cup opening. When the stream breaks, stop the watch and record how many seconds have passed.

Now you have the data to determine if you are at the coating manufacturer’s recommended spray viscosity for your spray gun.

If you do not have a factory viscosity recommendation, and you have to try several viscosities on your own, keep a spray gun settings journal “Coating Black Book.” Make notes and record your successful viscosity and coating temperature along with your gun settings for reference.

Viscosity summary

Sure, viscosity can be a pain to get right, but it can mean the difference between a great finish … and a not-so-good finish. Rather than waste time and materials, using these tried and true methods for understanding the right equipment needed, knowing the temperature, and recording can ensure a perfect finish every time.

Lane can help

Still uncertain how to tell which products help you achieve a great finish or have questions about viscosity? We’re here to help your business. You can always contact us, telling us the spray equipment you’re using, your material, and temperatures. We’ll have recommendations.

We’ve been helping customers like you get a perfect finish for more than 30 years. Plus, we can recommend abrasives, adhesives, and a variety of other products that help achieve a great finish.

It’s why Lane has been in business for six-plus decades.

Taylor Belmer
Author: Taylor Belmer
Taylor Belmer is an SEO Analyst and Copywriter at Lightburn, a digital agency partnering with the team at Elevated. She has 10 years of experience in the copywriting space, working with Fortune 500 companies, agencies, and small startups. In the past 5 years, she has grown her focus in SEO, UX, and digital marketing. Outside of her computer and the digital space, Taylor enjoys reading a good book, drinking coffee, trying new restaurants, and being active in the gym and outdoors.
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