A layer of Sleeper Grime/Frame Dirt mixture was prepared for airbrushing the sides, using more than the usual amount of white spirit. I wanted to be able to apply very thin layers, even thinner than my usual approach. Why a mixture of the two colours? Laziness, basically. My pot of one was empty and I couldn't be bothered to find another. The two colours are so similar that I didn't think it would matter.
The whole of both sides and ends was given a thin application of this mixture and then a wide flat shader brush was used to drag it down the tank sides before it dried compeletely. This was to give an impression of dirt being carried down the side by rain and generous tank filling exercises.
It worked better on one side than it did on the other.