Re: Wall
We have a bridge and wall that is legal to graffiti. Sometimes messages stay up for a day, sometimes it's a week, but invariably it's painted over in short order.
People have used it to raise awareness of an issue, propose marriage, celebrate a victory, or just paint something unusual or beautiful. From time-to-time, the city power washes it to remove the paint. It's considered iconic now.
Look how many layers were exposed when it was sprayed continuously in one spot. The oval is about 8 inches across and every ring represents a time in which the bridge was painted.