Wow Ron that engine has seen better days!!!
Not that I am any kind of expert on loco weathering (having only attempted one), but I would think the light spray of dust (from the bottom) and soot (from the top) should be the last coat.
Engines rust, water and steam leaves scale, paint peels off the smokebox. These things happen over time. The dust kicked up from the rails, and the soot fallout from the smokestack build up on top of these other things, and are often cleaned off between trips. So they should go on last.
From a modelling standpoint, I think this would have the added benefit of blending some of the weathering, so that it's not so stark.
Just a thought.
Val