You have a point, but in that case, I think your character would not be able to control metals that efficiently if they were smelted and mixed with other type of metals, like steel. I think aluminum and brass are ok, since they are natural, but they are usually found in small scale appliances or musical instruments. I think the metal power largely depends on if its natural or not, though. :/ That's just my take on it. It's hard to find a way to make it work because there is always going to be some hints of natural metal in almost everything, I just don't know to what extent the GM is going to let you control them, really.