Provided are stacks including CMC structures and capping layers deposited on surfaces of these CMC structures. Also provided are methods for hermetically sealing the surfaces of the CMC structures with the capping layers. These stacks may be used to construct walls of radomes that enclose antennas and other equipment of aerospace vehicles. The capping layers may form smooth external surfaces of the radomes and may hermetically seal the underlying CMC structures. The dielectric properties of these stacks may be configured to minimize interference with operations of the antennas and other equipment deposited within the radome.