In my opinion there is only one reason to thin paint for airbrushing, and that is to lower the viscosity so that the airbrush can atomize the paint properly. Given this, if the paint is already spraying properly and it is covering the model properly then there is no reason to thin it....
Thinning requirements vary for oil-based versus latex-based paints. When to Thin Paint There are a few reasons you need to thin the paint in your paint gun. If your paint is spraying unevenly or leaving the gun clumpy. If there appear to be lots of lumps and bubbles in the paint, ...
ADJUVANT LIQUID FOR SPRAYING, WASHING, DEGREASING, STAIN REMOVAL OR PAINT THINNINGLiquid intended to serve as spraying aid for phytosanitary products, for degreasing, washing, or stain removal, characterized in that it consists of a mixture of: a) a water-soluble wetting agent, in proportion from...