What are you using , Tamiya acrylic retarder or an aftermarket one such as golden or another artist supply store bought one . Usually it is be very conservative , add a bit and try it , if the paint dries too fast add a little bit more . It is not necessary to add retarded but it does improve the paint flow , be from an airbrush or a regular brush . Experiment and try it out .

This paint retarder will make acrylic paint dry slower, enabling a smoother finish. It can be used up to a 1:10 ratio with Tamiya Acrylic Paints. It helps to lessen the chance oｆ painting a grainy surface on your model. Also, it is ideal for painting glossy finish surfaces. 1 bottle contains 40ml.