Nice catch in Castle Douglas...
See in particular bottom right of shop front Castle Douglas Street Art
, on Flickr
It's such as shame that the majority of vandals and graffiti merchants can't stop their nonsense, and give up on preaching to the world they have some god-given right to spray, paint, or scribble on any piece of clean wall, or door, or street furniture on the basis that by being 'clean' it has been gifted to them as their canvas.
I know my neighbours and property owners nearby don't thinks so, as shop-owners have to clean or paint over the vandal's work, and I wouldn't like to watch the violent scene if some of the folk along the road, who have fitted white 'up-and-over' garage doors facing the street, ever met the generous 'artists' who donated their work on those doors 'for free'!
There are plenty of abandoned and decaying places they COULD go, but for most of them, they're only interested in being able to point at the damage they did, and proudly boast to their mate "I DID THAT!"