The fluid vapor film, as a new type of outdoor ironware sealing material, has a great potential in the application to the sealing of large-scale outdoor iron cultural relics. A new fluid vapor film sealing material suitable for outdoor iron cultural relics in most northern regions was developed through experiments and was applied to the surface sealing of ancient Ming and Qing artillery pieces. The results indicate that the new fluid vapor film had a good sealing effort, with the uses of 50A acrylic resin as the main film-forming agent, L18NF as the vapor phase corrosion inhibitor, fatty acid salt as the rust inhibitor and 12311 as the auxiliary agent. Either a water-based solvent system or an organic solvent system could be used for the new fluid vapor film, but the organic solvent system was more suitable for outdoor conditions in winter in the north. A composite brushing method utilizing ordinary brushing and matte brushing was adopted, which not only had a good rust prevention effect, but also reduced the reflectivity of the sealed surface. The new fluid vapor film method has the characteristics of being easy to operate and remove, resistance to cracking, salt spray, moisture, heat and impact, thus effectively preventing rust on the surface of iron cultural relics.