Exposure to water is a key issue in the performance of multilayer coatings. It may take place in different forms, i.e. as rainfall, dew and humidity variation. Consequently, coatings will experience time-dependent water activity fluctuations. In industrial practice, coatings are subjected to artificial water activity fluctuations in weathering tests. Little is known about the connection between these tests and the reality experience by a coating. This article presents a theoretical investigation of the response of multilayer coatings to water activity fluctuations. This investigation is performed on the basis of a validated model for water transport in hydrophilic base coat/hydrophobic top coat systems. The study aims to understand how permeability and sorption properties determine the overall coating response to fluctuations. It is concluded that present accelerated weathering tests do not mimic natural weathering due to the response time of the considered systems, which are insensitive to rapid fluctuations.