With so many commercial buildings having steel roofs or siding, rust is definitely a major concern. Rust can quickly cause issues with the integrity of your building, causing water leaks, entry points for pests, and even catastrophic failure in extreme cases. There might even be some metal parts of your building that you don’t think about that could be a problem, such as the rebar inside of the cement of your floors or foundation. When rebar starts rusting, you can quickly have cement deteriorate around it, cracking open and allowing for more water intrusion.
Whenever you notice rust, don’t just dismiss it as minor and superficial, instead, you should take steps to remove it before it starts spreading. Removing every trace of rust, repairing any damage, and then sealing it up well with a rust preventing paint or something similar is the best way to keep your building standing for a long time.