I have added a couple of coats of both darker and lighter gray that was brushed on and dabbed with paper towels for a galvanized effect. A gear will go into the upper section. It has been primed gray and will looklike rusted iron.
I've distressed the surface and added faint rusted areas. This photo does not do this justice. The piece really appears to be gavalized steel. I'll airbush a few areas to add a bit more depth.