A method for applying a carbon-based reflective overcoating on a grazing incidence optical unit comprising a substrate and a coating of a high-density material chosen from the group comprising gold, platinum, iridium, palladium, rhodium, ruthenium, chrome and nickel or a low-density material such as carbon or B4C; the method comprises the step of treating the optical unit with a solution or gaseous phase containing at least one polymer precursor material to create the overcoating through absorption of the polymer material on the coating.