For those who just lurk here wishing they could weather their rolling stock, here's a simple and effective way I do mine and it's even in N Scale. A simple wash of black artists oil paint diluted with Mineral Spirits , brushed on and brushed 95% off again so all you have is a somewhat dirty body. Taking a small artists brush again with oil paint (not diluted) and this time with Burnt Umber dab your rust spots on as shown. Again it must be with a small brush to get those ribs and only the ribs with rust. Actually you'll need various size brushes to do your weathering as you'll need a wider one when it comes to rusting the top of the car. Use a Dulcet spray or something similar to seal everything since the oil paint won't dry unless you do. I used graffiti decals where you see them, of course that's an option.