Reverse graffiti is a street art technique to force the council to clean up the rest of the wall if they want your message to disappear.
The idea is to use a pressure cleaner and a stencil to reveal a message on a dirty surface.
English artist Paul Curtis aka Moose is one of the first street artists to make an art piece using the reverse graffiti technique. He discovered the technique at his dishwashing job, as he explains in his documentary.
Follow this link to the Instructables page dedicated to the project.
Reverse graffiti - Wikipedia article
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