uncured thermosetting resins are loaded with relatively high amounts of solid thermoplastic resin particles to form a resin precursor . the resin precursor is heat treated so as to produce an uncured resin composition wherein the thermoplastic resin particles become substantially dissolved in the thermosetting resin without causing cure of the resin mixture . heat treatment of highly loaded thermosetting resins in accordance with the present invention provides uncured resin compositions that are well suited for use in fabricating composite structures and particularly prepreg for use in lightning protection surface coatings .