A cured resin product obtained by mixing (A) a compound having an addition-polymerizable group and capable of yielding a thermoplastic resin by its polymerization and (B) a thermosetting resin to form a uniform solution or dispersion and then curing the component (B) while simultaneously polymerizing the component (A), a process for producing the same, a resin composition, and the use of the same. As compound A olefinically unsaturated monomers like styrene, acrylonitrile or (meth)acrylates, particularly perfluoroalkyl(meth)acrylates, are preferred. The thermosetting resin is for example a cyanate, maleimide, phenolic or preferably epoxy resin in combination with a suitable curing agent. The compositions are used for water repellent coatings or for sealing semiconductor elements.