@carrotnl Already it looks better to limit the mud layer toward the bottom of the vehicle! Nice job. As for the dust layer, I would use the black and white output of that texture as a mix factor for your car paint shader and a separate dust material.
So I would add another pricinpled bsdf (or simply diffuse bsdf), make it a medium-dark brownish-grey color (dust) no reflectivity. Then add a mix shader node, plugging in your dust bsdf and your car paint bsdf. Then use that black and white dusk texture as a factor to blend the two bsdf’s together.
Does that make sense?
