When using any acrylic paint, not only Poly-S, I start with about 2/3 paint and 1/3 thinner (half distilled water and half denatured alcohol). Then add two or three drops each of Airbrush Medium (retards drying in airbrush spray tip) and Kodak Photo-Flo to eliminate the paint surface tension that can cause orange peel.
You may have to adjust the paint/thinner ratio depending on the color and/or how thick the paint is.
Pete