A good way to clean!

British street artist Paul Curtis (aka “Moose”) began pioneering his form ‘Green’ or ‘reverse Graffiti’ in 2007. Curtis literally scrubbed his way to fame using giant stencils and high-pressure water hoses to wash reverse images (mostly of trees and nature) onto soiled city surfaces. It did not take long before the aspirant street artist began conquering the cityscapes of London, applying his vigorous selective scrubbing to more prominent walls and bridges. 

[Text extracted from Neil Coppen]

For me this is art at it's best, inventive, transient, accessible and working with whatever materials you have to hand. Brilliant long live the inventive mind.