Does the impervious weapon property make metal weapons immune to a rust monster's rust attack? If so, why isn't the impervious armor property treated the same way?
I would say Impervious weapons can't be effected by rust monsters, going off the as-written properties. It does seem weird that the armor quality with the same name is so much worse, though. Not only does it give less benefit, but it's eating up a +1 instead of just being a (low) flat cost.