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Various techniques exist to simulate dirt, fading, spills, paint wear and tear, rusting, etc.
The result is extremely different shadows, contrast, etc, that spoil the illusion the model is trying to create. I was disappointed to find that I hadn't saved many of the recent posts on weathering and detail painting. So, I tried to pull the basics out of my somewhat leaky memory in order to put this section together. I can describe the way I do it on my aircraft kits. Basically, a wash can be made with any sort of paint which has been sufficiently diluted so that the pigment can run freely to all the crevices on the model.