Hey there aenimosity.
The building itself is supposed to be stone, with metal "equipment" built in and on it... It's basically a generic building that I just started and finished during our friday night club, so it was something of a rushjob, just to get something on the table. The rust on the grey area is basically the machinery leaking, the liquid mingling with the rust, and then spilling out over the stonework. I plan to add some greenish mould as well. So yes, it's supposed to be stonework. I know that metal equipment built into a stone building (especially with this shape) is a little bit strange, but hey, it's "fantasy", and I reaaaaally just wanted to get something finished for a change.
If you still feel I did something wrong, perhaps you could elaborate by showing me some pictures of your rusty farm equipment... I am no expert on rust, so the things I do are really just things I think look good and things I think (but not know) are at least somewhat realistic. But please, if you think you can help me paint this piece of scenery any way better than I did, it would please me if you could help me out!