Riding the train from Boston to New York, you see quite a lot of abandoned industrial buildings and equipment, and therefore a lot of tagging.
Preventing graffiti presents a really interesting challenge to the world of material science, which is quite a fascinating discipline in and of itself.
One approach to graffiti prevention is to create building materials with high “surface lubricity”, which resists markings from sinking into the material and makes it easier to simply wipe off whatever’s been applied.
A second try is “dimensional surface pours” for concrete, i.e. irregular surfaces which are simply much tougher to write on.
A third approach – which in some sense is an extreme version of the dimensional surface – is to use “vegetal surfaces”, i.e. let plants grow over the walls.
Rusty iron apparently has very poor surface lubricity.