The ROSAT Wide Field Camera, launched in June 1990, uses large area (50 cm2) thin film (typically 0.5 micrometers thick), band pass filters to select different extreme ultra violet wavelength bands. One of the filters consists of a substrate of the plastic polycarbonate, Lexan$DAG, interleaved with carbon and is thus susceptible to erosion by atomic oxygen in the ROSAT low earth orbit at 580 km altitude. The filter was protected against this erosion mechanism by using a thin overcoating of boron carbide. We describe the boron carbide coating process, the technique used to minimize the heat load on the fragile plastic foil, and the need for an additional adhesion layer of carbon. The chemical composition of the boron carbide as evaporated material on glass slides has been measured using several surface science techniques as well as by analysis of the soft x-ray and EUV transmission of sample foils and completed flight filters. Additionally, using ion and atomic oxygen sources, the effectiveness of the coating has been evaluated by laboratory measurements on sample foils.