Composite materials have high physical and mechanical properties, but their widespread use is hampered by a lack of tightness (impermeability to moisture and gases). Mainly this is due to the stress-strain state of the composite structure at the micro level as a result of a comprehensive extension of the polymer owing to the adhesion of the matrix to the fiber. The development of cracks and epoxy binder swelling causes its shrinkage and thus, may change the size of distribution and the internal stress patterns in glass fiber wool and lead to a partial disruption of adhesive bonds at the interface, formation of pores, cracks and other defects in the boundary polymer layer. Finally, it can be the reason of its partial detachment from the fibers’ surface. It should be noted that such changes depend mostly on the chemical structure and composition of the polymer binder. Watertight composite material development increases the possibilities of its using in many industries, including aerospace, shipbuilding, etc. In this paper the water resistance of polymer compounds was investigated by comparing the properties of the samples with modifying compounds based on epoxy resin. As a part of the study water saturation curves were determined for various compounds and curing agents, the optimal concentration of additives were measured. The results demonstrate the possibility of using of polysulfone as an additive in epoxy resin, improving the water resistance of fiber reinforced plastic.