I would like to add to this, after something I noticed tonight with my own model.
I was giving a dirt wash to my car (1/24) using a thinned down Vallejo Air, applied with a brush. Now my model is plainted with car paint, and has an aerosol glossy clear coat on top.
I noticed that my previous wash had dried in a fashion of leaving a circular tide mark and fading to nothing towards the centre.
My suggestion would be perhaps to put down a little clear coat. Mix up a paint of suitable colour and just gently put a nice blob down. Hopefully held spherical by surface tension and allow to dry. Another coat of clear, a different colour in a smaller blob and procede until the middle?
This is just a working theory of course but it makes sense in my mind
-Matt-