Just to add,
use AO (ambient occlusion) switched on in Indirect Illumination, this will add some "dirt" to the edges where they meet other objects
make a dirt material, this will do something similar, is deeper controllable an will only show up on these certain material (it's a little tricky but maybe good for your brick wall)
Good luck
btw: it's more controllable if you add your lines or layer in a photo editing software