1) Painting plastics can significantly impact their mechanical properties in complex ways that are difficult to predict without testing. 2) An experiment tested the effects of painting on the tensile modulus, tensile strength, strain at break, and impact strength of four common plastic substrates. 3) The results showed both positive and negative statistically significant effects of painting and primers on the mechanical properties of the different plastics tested. Therefore, testing is needed to determine the effects on a given plastic.