I'm tinkering with composing N scale building elevations on card stock. The elements are taken from a free download. The weathering/aging of the sign was accomplished by layering it with texture from around around it. I use the Canon Photo Studio 5 clone tool at 30 to 50 % opacity. I really like the brick texture available on this paper model. Individual bricks are not really discernable in N. This mottling I think does it. Keep in mind the 3 foot 10 second rule.