Thinning acrylic paint for airbrushing is a process that is highly subjective. There is no one recipe for thinning paint that will satisfy all uses. The amount of thinner necessary will depend on the thickness of the original paint, the pressure used in the airbrush sprayer and the area that you want to cover with paint. All of these factors will affect how much thinner to add. However, if the paint has the consistency of mud, rather than the consistency of pancake batter, then you will have to thin the paint before even practicing the test spray.
Determine the pounds per square inch that you want to use for the project. When painting larger areas, you can use a higher psi. Use a psi around 20 to 30 for larger areas. For smaller areas, a psi of around 10 will often produce better results than higher pressures.
Thin the paint to the consistency of thin cake batter using the airbrush medium before adding the paint to the airbrush sprayer. Stir the materials together by shaking them up inside an airtight container.
Test the paint in the airbrush sprayer. It is likely that you will not have to reduce the paint any further, or only by a slight additional amount when used at higher pressures. The spray should cover evenly without drips, splatters or overspray.
Add a small amount of airbrush medium to the paint at a time using an eye dropper. Continue to test the mixture until you are satisfied with the results. For low pressures, the paint should be about the consistency of whole milk. Keep track of the amount of airbrush medium to paint so that you can know exactly how much medium to add per psi pressure rating when airbrushing in the future. Write down the ratios for future use.
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