I pondered how to weather them for quite a while. Airbrushing is the obvious method for such a large area, but I do that in the garage to keep the smell under control and didn't fancy lugging the arches down there and filling up the workbench.
Instead, I had a go with sponges and emulsion paints. First on a test piece and then on the arches proper in an orgy of mucky-fingered painting.
I'm using high-density foam, two shades of grey and some black. Dabbing the paint on seems to work, but a rolling motion works well for blending. I start with black and then move to grey. Each colour has its own sponge square.
Working on a small area - about a foot long - at a time means the colours blend on the brickwork. I sometimes went back to black after putting on grey as it seems to soften any edges.
The paint dries to a slightly different shade, something you only lean with practise. Even after the test pieces, I started on the back of my viaduct!
One advantage is that the colour dries perfectly matt. The downside of this method is it's not easy to get paint right into the corners, but that will be finished with weathering powders later on in the build.
For the moment, I'm very please with the effect. It might be messy and not that fast, but it's certainly less smelly than spraying.