Olivier van Herpt invented a machine called the Time Writer. It allows any graffiti artist in training to get their message out to the masses with the push of a button. An Arduino controls the action here. The Arduino pulses a number of relays which open and close paint nozzles. The paint mixture is pressurized using a re-purposed water fire extinguisher which conveniently has a valve just like a car tire. I think this would be a very effective guerrilla advertising technique if it was hidden under a car and used water soluble paint so that your message only lasts until the first rain. Â