We generalize the four-flux radiative transfer model to the case of a multilayer medium. A concrete application, that of the study of the optical degradation of white paint in a simulated space environment, is presented. The degraded material is decomposed in a damaged layer and in an unaffected layer, and we assume that the degradation is due to a variation AK of the imaginary part of the refractive index in the damaged layer. Then we find an empirical law for variation AK with dose, taking into account possible saturation. (C) 2001 Optical Society of America.