A washer, a fastening assembly including the washer and a composite structure including the washer sealing the assembly for internal lightning strike protection. The washer includes one or more concentric ribs that are dielectric rings on both sides. When used (e.g., with a nut and bolt) internal to a structure, especially a composite structure, the washer seals the fastener hole (i.e., that the bolt passes through) and contains any sparking and hot gasses that may arise in the fastener hole from entering the structure.