I've been looking at some material armor combinations for a character I plan on creating, and I can't quite figure something out. Certain materials, such as Darkleaf or Mithral, say that armors made of them are always masterwork. Does this mean that the -1ACP from a masterwork armor stacks with the -3ACP of Mithral? Personally I don't think it does, but I can't find a rule saying that they don't stack.
They don't stack; it's already built in.
http://paizo.com/threads/rzs2qy3g?Mithral-Armor-is-always-Masterwork#1