Is there an efficient way to add a weathering effect to an object? By weathering, I am thinking of an overlaid thin layer of mottled grime on a sign or a car, maybe a little rust here or there.
There are many video-tutorials for this topic on Youtube and (answered) questions here in BSE. To get you started I selected some of them for you:
The approach I settled on was relatively simple: because the underlying object was a rectangle, I created a plane of the same dimensions and superimposed it on top; loaded a black and white image for texture and then adjusted the transparency. It can be found here.