I worked on several projects involving the use of Cor Ten steel. This steel is designed to develop a surface layer of rust, forming a protective layer for the steel underneath. Once this protective layer of rust is formed there is no further degradation of the base metal. A frame made out of Cor Ten would last a long time without any special treatment or coating.
https://www.ssab.us/products/brands/ssab-weathering-steel
https://www.ssab.us/products/brands/ssab-weathering-steel