Tagging is the term given to a type of graffiti which consists of people spraying their 'tag', a device which represents them, onto public spaces. I read in a reputable journal of anthropology* that it can be considered a form of response to today's 'brand' culture: in a world where every product has a brand people react by creating their own brand and placing their logo everywhere.
Tagging is usually annoying to whoever owns the space it occurs on, as well as to whoever has to remove it. Eventually walls may become so tagged that it is hard to discern the original surface amidst the colours.
This invention does not seek to stop this completely (which seems nigh-impossible) but to turn it against itself. The object itself is similar to a spray can, but rather than spraying paint it sprays some kind of cleaning substance (possibly sodium bicarbonate). It might have to be larger than a normal can to accomodate higher pressure spraying.
Now, instead of tagging with paint, the user tags with the cleaned original surface. Once the entire surface is clean enough that further 'inverse tags' are invisible, the space owner rejoices before a new wave of conventional tags appear.
*Alright, I'll admit it, it was a Dinosaur Comic.-- dbmag9, May 22 2008 (?) A bit like this advertising idea http://if.psfk.com/..._cleani_extract.phpBut for graffiti. [theleopard, May 22 2008] Dirty Street Advertising http://www.dirtystreetadvertising.com/With dirt, not graffiti; otherwise very much as described. [jutta, Feb 09 2009] I've added a link but can't view it at work for some reason, but the google hit looked like what I was looking for. Basically, a geezer went on Dragon's Den with an idea to steam-clean stencil adverts on the dirty pavements in cities. Similar concept like.-- theleopard, May 22 2008 It's baked here in Manchester. There are adverts steam-cleaned onto paving, and some noncommercial art 'cleaned onto' dirty concrete walls. I'll try and get some photos.-- Srimech, May 22 2008 [+] because it's a good idea, and not well-known.-- shapu, May 22 2008 The Kony 2012 campaign suggested doing this to dirty sidewalks using bleach spray and a stencil cut from the bottom of a paper bag, IIRC.-- notexactly, Apr 23 2019 random, halfbakery