The first post-painting phase of my weathering process is usually a wash, I use one of those readily available products from Mig or AK for this purpose.
I apply it heavily on the surface and then remove it step by step by dabbing the surface with brush with little bit of enamel thinner in it. It is a slow process, but gives me a lot of control over the final look. I always remind myself that once dry, the overall look of the wash effect is less dramatic compared to the way it looks while still wet. But I would rather err on the side of having less of an effect in one step and having an option to repeat it, rather than do something that cannot be easily reversed. Also since this is the first step of the weathering process, it prefer to go light, as there will be (quite) few more steps following.
The whole purpose of wash is to break the uniformity of the painted surface and add the first layer of dirt, but partially the wash also acts as a pin wash, though it is always good to follow up with a dedicated process of pin wash later on.
I have also applied mud solution in the space behind the wheels. Almost none of it will be visible once wheels are mounted, but I wanted to test the look of it. The whole purpose is to add bit more “mud” volume before application of pigments.