February 26, 2023

How to thin water-based polyurethane for spraying

Have you ever wondered how you can thin polyurethane for spraying? Look no more, as this article will take you through the process. Thinning polyurethane helps you to get an even and smooth finish. The polyurethane in spray cans does not need thinning because it is as thin as possible. However, if the polyurethane does not come in a spray can, you must thin it first and then load it into the spray gun. It is too thick to spray. This article will take you through the process and reasons behind thinning before spraying polyurethane and how to use water-based and oil-based polyurethane.

Determine the appropriate viscosity

After confirming the coating is thinned, write down the time the material took to break the fluid stream. That would give you your viscosity. With the viscosity range written down, start setting your paint application equipment to achieve the proper appearance finish you want. 

If you cannot achieve the desired finish, try to see if your viscosity is too high and add thinner as indicated on the technical sheet. With the viscosity written down and the equipment settings adjusted, you will have a better polyurethane for achieving a paint finish.

Work on the appropriate viscosity of the polyurethane before thinning it. Viscosity is a measurement of resistance to flow. The correct viscosity ensures the polyurethane thins correctly and sprays evenly. The higher-viscosity material requires more energy to atomize, whilst the low-viscosity material may be over-atomized. 

Wasted material due to overspray, dry overspray, too rapid drying impeding flow out, and lower dry film thickness per coat is what you get. The more the viscosity of the material increases, the more the capacity of the fluid to flow easily will decrease.

Consult the manufacturer’s recommendations or use a viscosity cup to measure the thickness of the polyurethane. The rule of thumb in the paint industry says that a viscosity of 100 cps provides spraying, brushing, or rollers (Paint.org).

Use a cup to measure the viscosity of your polyurethane. The paint Viscosity Cup is for measuring the viscosity of paints and finishes. Using a stopwatch in one hand, hold the cup by the ring on the top with the other hand and dip the cup. Dip it into the coating. Pull the cup up and out of the coating. Use the stopwatch to monitor the time the cup gets clear. Note the time and compare it with the stated time recommended on the product data sheet. Some charts will help you find the equivalent reading for your cup. 

Gather the necessary tools and materials

To thin oil-based polyurethane, gather mineral spirit, naphtha, or lacquer thinner as solvents to thin out the polyurethane layer. Use a measuring cup for the perfect ratio of polyurethane and mineral spirit before your polyurethane becomes too thin to perform better. Use a clean stick to stir the polyurethane and mineral spirit mixture. Stirring gives you a consistent mixture. 

Thin the polyurethane

  1. Clean the glass jar. A glass jar is where polyurethane meets mineral spirit, and a reaction happens. The chemical reaction should take place in a clean environment without any interference. Clean and remove dust, stains, paints, and other residues in the glass jar using warm soapy water.
  2. Pour a sufficient amount of polyurethane into the glass jar.
  3. Add mineral spirit or paint thinner to the glass jar. Maintain the ratio of 1:4. Four parts of polyurethane and another for the spirit.
  4. Use a measuring cup to get the ratio of polyurethane and mineral spirit. Pour ½ cup of mineral spirit into 1 gallon of polyurethane. Add paint thinner or mineral spirit a little more to the polyurethane, depending on the type of application.
  5. Stir the mixture using a clean stick and stir the polyurethane and mineral spirit mixture for about 10 minutes. Maintain even pressure on the stirring stick and follow a slow stirring motion. Stirring too fast can cause bubbles in the polyurethane mixture. If the air bubbles are trapped inside, they never leave after polyurethane coatings. At the end of the stirring mineral spirit-polyurethane mixture, air bubbles disappear.
  6. Test the Polyurethane on cardboard or a wood piece with a spray gun. Wait until the polyurethane dries to see the colors. Check for any imperfections, bubbles, or color variations so that you have successfully thinned the oil-based polyurethane. If you feel polyurethane is too thin, add polyurethane a little more to the mixture, stir and do the test again. If the polyurethane is still too rough, add mineral spirit a little more to the mixture, stir and do the test again.
  7. Load Polyurethane into the spray gun. Make sure the polyurethane is thin for a smooth glossy finish without making any blotches on the surface. Expect excellent results.

Test the viscosity

Test the viscosity of the thinned polyurethane to ensure that it is appropriate for spraying. Use a viscosity cup or spray a small test area and observe the flow of the polyurethane. For better viscosity to your polyurethane,  there are factors that you need to control. These are temperature, speed, and time. The most important factors to know or control are the following:

  • The temperature may affect the viscosity value. The temperature increase will give lower viscosity values. Compare samples’ viscosities or to be by standard values. Speed also influences the viscosity of your polyurethane.  
  • Time is the third variable to control for your polyurethane product because viscosity tends to decrease when the shearing is longer than if it is short. It could affect the process and trouble also the accuracy of viscosity control.


In conclusion, different types of polyurethane require different thinning agents for better results. Oil-based polyurethane can be thinned with mineral spirit, lacquer thinner, or naphtha. Water-based polyurethane can be thinned with water or denatured alcohol. Thinned polyurethane seals the wood better and prevents surface blotches and streaks. Thinned polyurethane gives fewer texture bumps and dries faster. Follow the mentioned steps to thin water-based polyurethane for spraying, and seek professional advice if you have any doubts or concerns.

David D. Hughes

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