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Grey or other colored rectangles are the answer to graffiti in most North American cities, giving rise to some interesting new patterns of wall-painting.
Matt McCormich made a film about it. Narrated by Miranda July, “The subconscious art of Graffiti Removal” is a rather subversive take on this, explaining that Graffiti removal is “one of the more intriguing and important art movements of our time.”

Here’s 6 minutes of the film:

Images 1,4 and 5 from unurth.com, visualslushpile.blogspot.com and freeassociationdesign.wordpress.com