I've heard that daVinci's Last Supper began deteriorating only months after it was finished. Apparently the north wall of the monk's refectory at Santa Maria delle Grazie in Milan was not entirely hospitable to the oil-and-tempera based medium the master used to paint the masterpiece.
Ironic then that this wall scribble is maybe a decade old on the southern wall beneath a railroad bridge going into Lancaster. While it rarely gets direct rain or sunlight, it's otherwise completely exposed to the temperature and humidity shifts of our part of the world. Yet it's still as bright and uncracked as it came out of the spray cans. Just as roaches will survive an atomic blast, grafitti will probably build the stories of the Fourth Millenium's archeologists. Don't you wonder what they'll conclude about us from these sort of things?