The mural on the tank has been lacquered and flatted ready for detailing, this entailed water based washes to add extra depth and to kill of the starkness of the white, and these washes have to be applied very carefully, too much and its overkill, too little and the whole thing will look dirty, as if somebody had handled the thing with dirty hands.
Once the washes have been applied the artwork can be highlighted and super detailed with either a brush or pen using numerous shades of grey.

Nearly finished with the texturing, without Lacquer the bones look as though they have just been exhumed after a hundred years or so and are just secreting age.
I find weathering and aging a seriously rewarding challenge as you can add or subtract detail with a simple brush stroke, subtle use of oil paint can add yet another level detail, by dry brushing gentle controlled textured blends cab be easily achieved without much practice, using cotton buds instead of a brush can also add extra interest.