A long-lasting barrier protection at damaged sites of coated system can be achieved by means of an active response of the primer. This may be attained through the release of reactive liquids from dispersed containers, the increase of the local mobility of the polymeric network, or the expansion of (dispersed elements in) the primer [1]. Such self-healing mechanisms provide an excellent strategy towards repeated and sustained self-repair properties and, additionally, are not incompatible with the inclusion of passive elements within the primer to produce a corrosion inhibition response. This work focuses on the modelling and simulation of expandable coatings resembling systems that contain thin clay-based interlayers [2]. However, a similar methodology could be applied to treat the hydraulic growth of inorganic grains in PPS coatings [3]. The expansion of a theoretical porous plastic primer is triggered by the ingress of environmental moisture. The volumetric growth of the primer into the crack is investigated under different humidity, temperature and atmospheric pressure conditions resembling different geographical locations (coastline vs. inland, changes in altitude and latitude, etc). Furthermore, the reversibility of the expansion process is investigated through hysteresis cycles resembling the change of the environmental conditions over a time lapse of several hours up to days. The self-sealing capacity of the coating is estimated from the maximum crack width that can be filled. A full characterization of self-sealing efficiency is given in terms of key design parameters such as porosity, permeability, sorption behaviour, swelling range, shear modulus, layers widths and adhesion properties.