I'm not familiar with Blender. Sorry! But you could check whether it's possible to paint directly on the model.
I use Substance Painter, and love it. It allows me to paint directly on the model, as well as use different filters that makes use of information from ambient occlusion, curvature and normal maps in order to add dust, grime, dirt etc. on the natural places.
Other similar tools are 3D Coat and Mudbox, all though I'm not familiar with their toolset.