Good Scenic Artists are defined by their skills to paint reality. Whether its matching a colour or texture or aging a sign; the object is realism. You'll see them studying bits of rock and brick, they have samples of stuff all over their shop. Don't throw out that chunk of broken tile! Its part of their kit!!!
Its easier to screw up aging than anything else. Anything else can ultimately get passed of as fake decor. Fake brick, cultured stone, faux wodgrain.
But a poorly aged Set is a shame. A beautifully executed Set can be brought down to fakery by one untrained Scenic. Over-aging is one of the most common give-aways on screen. Its a really hard craft to master. Thought and sense also plays a role. Where would it be most rusty? Where would the rot form from?
The new cameras also play a role. Aging needs to be less obvious, yet at the same time, white signage still needs to be brought down or it will be too hot.
I am a big big fan of photo reference. It doesn't always reach the Scenics doing the job, however. Sometimes good photo reference just ends up amidst the pile of Art Dept and Office paper, because thats just the nature of the biz.
If its really important, or you don't know your Scenics, go meet them, bring them the photo reference. You'll get huge respect from them that you even consider their efforts relevant, they are so far down the chain.