Invention: Graffiti warning system
New Scientist, 10 September 2008
http://www.newscientist.com/channel/being-human/mg19926733.700-us-army-is-breaking-rules-to-make-terrorists-talk.html?feedId=online-news_rss20
"Paint-based graffiti can usually be removed relatively easily from buildings, bus shelters and other street furniture. But graffiti that is scratched into surfaces such as Perspex is much more difficult to cope with and usually requires the entire surface to be replaced at great cost...."