One important difference between pure pigment, like this rust, and ordinary paint, is that paint contains both pigment and a binder medium, which keeps the paint together. Applying the rust wash and letting the water evaporate leaves a this coating of pigment that can easily be rubbed off. So before I go much further, I want to spray the base with a sealer to help keep the rust pigment in place. For this I used Dullcote. (I also masked off the sides of the base first, where I didn't want any dullcote to end up.)
In the next picture I have also added a black wash, composed of Thamar black, water, and fluid matte medium to help begin forming the shadows.
At this point I have a fairly good base from which I can begin building up the rusted iron that will make up the majority of the scrapheap, so I decide to add some colors the other major base elements. The tree gets painted a dull gray-brown, the large gear and the ends of the electric coils get greenish bronze, and the coils themselves are painted gray. I also paint the warjack a faded dull red, but as this is an old scrapped 'jack, I leave plenty of gaps in the paint to represent areas where the paint has chipped or flaked off.
I have a few more details to add before I begin assembly. I want to make sure the shadows are in good shape, so I'll do a few more black washes, this time targeted at key areas of shadow. (I'll do the highlighting after assembly, since the areas I can't reach because of the assembled base elements are supposed to be in shadow anyways.) I also want to make sure the tree and the rope on it are fully painted, since parts of it will be hard to reach with the wire mesh in place.