I would say, search for reference images of rusted objects similar to the one you are working on. Generally speaking rust happens evenly on unprotected iron surfaces exposed to moisture. So the rust pattern depends on - how does any protective layer on the metal looks like (e.g. scratches on paint, chipping paint, ...) -…
You could spend hours and hours tweaking procedural masks or making super complicated and slow working node graph in Substance Designer . People manage to reach amazing results with that out of pure sport like approach. Still each new seed/generation would be slightly off of what you really want. So in my experience you…