How Do I Calculate the Volume Solids of a Coating After Adding Thinner?

In order to calculate the dry film thickness (DFT) required to attain your desired Wet Film Thickness (WFT), you’ll need to use the following formula: WFT= DFT/VS

Conversely:  DFT = WFT x VS

WFT- Wet Film thickness     DFT= Dry Film Thickness   VS= Volume Solids as a decimal (60% = .60)

You’ll find the volume solids listed on the coating manufacturer’s technical data sheet. NOTE: you may also see the solids by WEIGHT listed. Unless you need to know the weight of the dried coating that you are planning to apply, you can ignore the solids by weight. This would be useful if you are coating an airplane, but not useful if you are applying a tank lining.

If you are planning to thin the coating prior to applying it, how would you adjust the volume solids figure? Let’s do a story problem. I used to hate story problems in school, but let’s give it a try.

Walter is applying a two component urethane coating with a 60% volume solids of the mixed coating as indicated on the manufacturer’s data sheet.  He’s planning to mix a one gallon kit of the urethane and then add a quart of thinner to attain the desired viscosity prior to application.  After adding the thinner, what is his new solids by volume (VS)?

Some common guesses are below:  

  1. A)     .60 – .25 = .35   

This painter took the 60% VS and subtracted 25% VS because he added a quart of thinner, which is 25% of a gallon.

  1. B)     .60 x .75 = .45

This painter took the 60% VS multiplied it by 75% because he added a quart of thinner. He subtracted the quart of thinner from the full gallon volume solids, leaving 75% of the manufacturer’s original VS.

  1. C)     .60 divided by 1.25 = .48  

This painter took the 60% VS for one gallon and then divided by 1.25 gallons because he added a quart.

And the answer is…

C = .48, or 48% volume solids.

If you got this one wrong, don’t feel bad. Most people get it wrong. Worse yet, most people use the original VS regardless of the amount of thinner that they added and then they wonder why their WFT and expected DFT don’t match up. In that case, low DFT readings will probably be observed. Just remember that the volume solids is the amount of solids in a GALLON of a coating. In an unthinned gallon, the volume solids is VS/quantity of 1 gallon. When you increase the quantity of the mixed coating by adding thinner, you’ll increase the quantity. In the example above, just divide the original volume solids by the new quantity of 1.25 gallons, and you’ll get the correct answer.

If you have questions about this or industrial coatings, please call or email Carter anytime.