Here is a stab at some theoretical analysis. I have produced a quadrant based on two variables:
Here is how I see each quadrant:
The money pit
The top left quadrant is the money pit - a site that is hit regularly by graffiti, and requires a regular (and expensive) cleanup. These sorts of sites are crying out for a different approach - better lighting or site lines, a change in design, introduction of creepers on walls etc etc. Insanity can be defined as doing the same thing over and over again and expecting a different result. Painting or cleaning a site week after week and changing nothing and then expecting graffiti to not come back is insane.
If you're in this quadrant, you need to be doing something differently. I think the sound barriers alongside the City West Link were a good example of this - they were being hit again and again, requiring frequent cleanups, until the RTA put in more creepers and plants and covered the bare, graffiti prone walls with foilage.
The dump is the bottom left quadrant - a site that is frequently vandalised but very rarely cleaned. It's the kind of place that no one would want to live in. The area underneath the motorway in North Strathfield is an example of this - I doubt some spots in that area have ever been cleaned. The dump is probably an area of falling property values, a high fear of crime and low pedestrian traffic - especially after dark.
The bottom right quadrant is the garbage bin of sloth. This is a location that is very rarely hit, but also very rarely (if ever) cleaned.
My experience over the last 6 months has shown that there are many, many sites that I would call "targets of opportunity" for vandals. Like the rest of us, I think vandals have patterns and are creatures of habit. They take certain paths again and again when they travel from here to there, and most graffiti will be found along those paths. (If you look at a paddock full of sheep, there'll be clear paths worn into the field where the sheep prefer to travel). These paths may be on their trip from home to school, from home to a friends place, or to the shops or a leisure activity (like a skateboard ramp).
However, from time to time, the vandal will break their pattern and find themselves in a different area, and because they have a compulsive need to leave their tag, they'll tag targets of opportunity.
They might never go back to that location. However, if the property owners are slothful, the tags will hang around for years.
My reasoning behind this is that I have spotted many sites which have "lonesome" graffiti - one site has been hit, but none in the surrounding area. I've asked for that site to be cleaned, and six months later, it is still clean. Either vandals are leaving it alone, or they simply rarely if ever get down that way.
These sites are the "low hanging fruit" of graffiti. Once they are cleaned, they should stay that way for a long period. The problem we have is that some people have a mindset of "why should I clean it - the little buggers will be back tomorrow to do it again". No, they won't - because your site is not on one of their regular paths. You were an unfortunate target of opportunity, and it is unlikely they will be back soon.
Zone of pride
This is the top right quadrant, which covers sites that are infrequently hit but quickly cleaned. This is where a property is hit at night, the owner sees the graffiti the following morning and has it removed that day. It might be the first thing they do that morning, before anything else. Tags on these sites have a lifespan of less than 12 hours. Owners are vigilant and ruthless about removing graffiti as rapidly as possible.
The thing about these sites is that only two groups of people know about them - the vandals, who see their tags gone when they walk past again in the morning, and the property owners who are removing them. The tags are gone so quickly, members of the public walking past on their way to work would never know that some were sprayed there the night before.
Ideally, this is where we want to be. If we can't put a stop to graffiti being applied, we can at least get property owners into the mindset of rapid removal, and surface treatments that either allow rapid removal, or reduce the frequency of vandalism.