I would go with the way it was originally done..
I also assume you are using an airbrush...
However before you start, I would outline certain areas in black. To explain this better you may have to look in the armour or aircraft forum. It basicly a black line that is spray painted on the panel lines. In your case it would be on corners or areas where to parts meet such as the front of the hull, around the port holes, corners of the gun turrets and so on. (hard for me to explain,,,but in the other forums it is mentioned a bit and you can see it on many posts).
Then start with the base colour which in this case is the original grey colour match. Paint right over the black lines you did earlier. (go with light coverings and gradually build it up) This will eventually give you the basic color of the hull and the black lines will subtually stick out.
The next layer would be the fading effect which is the same color but lighten up. You may need to put several lighter shades progressivley till you get the effect you want.
Then I would start the weathering effect such as water streaking. Then you would add the rust and other paint chipping.
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