That is probably the main reasoning behind choosing Platinum. Sato's freaking rich, hell i'm 99% certain he's the richest guy in the entire avatar world. Who knows, maybe it could be a plot point and the Equalists had found a crater where a platinum rich meteor hit ages ago?
In reply to ZetaEtaTheta, #8860: To be tedious: That specific space rock likely did not have platinum in it as both the raw and refined material was a darker metal; platinum is grey and very shiny, even as an ore.
If the mechs and the giant metal blast door were only platinum plated and not 100% platinum, would metal benders still not be able to bend it solely because of the platinum content interfering with their bending abilities?
I don't think so. To my understanding it's really hard to metal bend without physically touching the metal. In the preview of the promise part 2 toph talks about floating coins but whenever you see big metal bending there's always physical contact.
........I GET IT!!!! but that does raise an interesting point. I really am starting to think that might be the case. I mean, we're already got cars and mecha- tanks. (to be honest, i think the benders should get RPGs as compensation. XP)