okay, the mysterious effect is none other than brushes in photoshop. this is how you make them, and apply them.
weathered brush:
what i do, is i find, take images, or scan things of something beat up, rust, asphalt, tape, torn or weathered materials, etc. you take this into photoshop and you cut out the area that you want to be a brush and paste it into a new document. then you desaturate the image to become black and white, play with the contrast so that some areas are completely black (basically photoshop will read the greyscale, black being full, white being nothing). after you are satisfied, and your rust, etc is the way you like, you go to define brush in one of the menus at the top, forget where exactly. this will now make a customized brush for you to play with.
now you have your brush, i usually have a couple of them that i use in combination with each other. for application i use the paintbrush tool (not the pencil) make sure you select your new brush and then make a layer and go play, which is what i did for the white layer behind the angel. i usually duplicate layers before i do something drastic, just in case it fucks it up. you then can take the eraser tool and select the same weathered brushes to take away from the image as well. i sometimes erase some of the paintbrush weathering if it is too much, you can also rotate weathering so that it looks the way you want it to.
that's basically all it is. i'm really into silkscreen and stencil aesthetics, so this works pretty decent. i've made some posters with this effect and people actually thought they were silkscreens standing right next to it.
here is another example of it.
