Chances are good that you already know what viscosity is, but you’ve probably never needed to put the definition into words.  Quite simply, viscosity is the measure of the thickness of a liquid.  More specifically, viscosity is the resistance offered by a fluid to outside stress placed upon it.  More viscous fluids are thought of as “thicker,” while less viscous fluids are usually considered “thinner.”   For example, syrup is more viscous than water, and oil is more viscous then vinegar.   Absolute, or dynamic, viscosity is the resistance a fluid offers to flow when placed next to another fluid moving at a given speed.  Kinematic viscosity is the measure of a fluid’s absolute viscosity when divided by its density.

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For the paint professional, viscosity determines how much paint should be applied to a brush or roller and how much paint is necessary to cover a given area.   Furthermore, the viscosity of a paint or coating will determine the appropriate calibration for sprayers and airbrushes.  For example, when planning how much paint will be needed to finish a job, it is important to know that a more viscous paint will cover more area than a less viscous paint. When air-brushing or using a sprayer, it is important to understand that thicker paints will require more air pressure to distribute evenly on the surface.  Finally, different temperatures of both the paint and the surface to which it is being applied can affect the viscosity of the paint and must be accounted for when determining how much paint will be necessary to properly coat a service.

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Examples of thicker, more viscous paints include enamel automotive paint, petroleum based sealers, and acrylic art paints.   These paints often allow very little bleed through from the surface and require less volume to completely cover the surface.  Thinner, less viscous paints include food color dye, watercolors, and lacquers.  These paints often require multiple coats to achieve the desired coverage of a surface.

Often times the viscosity of the coating or paint needs to be altered prior to beginning a job.  Changing the viscosity of a specific paint is rather simple and can be accomplished by taking a thicker paint and thinning it out using an appropriate additive.   For example, water can be used to reduce the viscosity of water-based paints, while alcohol or acetone may be used to reduce the viscosity of acrylic paints.

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For professional painters, understanding the concept of viscosity can go a long way in making sure the right materials and properly calibrated equipment are being used for the job.  While a thorough understanding of the physics of paint is not imperative to being an excellent painter, it never hurts to know how your materials will behave under different conditions.  Whether painting in cold temperature, or warm, dry conditions, or humid weather, understanding the physical properties of paint can help you do a great job every time.   If you are interested in tools to help determine exact viscosity click here!