I would paint the wash on all over, especially the edges of the plates and dab off the middle of the plates with a sponge. This concentrates the wash effect into the rivets, weld lines and so on.
There are loads of techniques for detailing armour and they can often be combined if you want to spend the time. In making this heavily weathered vehicle I did multiple goings over with different washes and dry brushings over a basic salt weathering technique.
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