I noticed something interesting that happens if you use RAW with Heat Metal.
It only affects creatures in contact with the metal, so someone wearing clothing under all their armor is completely unaffected by the spell, and their clothing is unaffected as well.
But if the targeted armor wearer grapples with someone, that creature will take the damage if they are in contact with the armor at all.
Also, what precisely is unmanufactured metal? The spell doesn't target that.
EDIT:Additional thought. What is the maximum size of something you can target with the spell? Would an entire metal castle count?
You could
try arguing that, but that'd be stupid, since almost no armour has metal in direct contact with skin anyway - they'll all have layers of padding beneath them, and indeed almost all of them describe armour as splint/maille/whatever on leather, or cloth. Wearing clothing beneath doesn't mean you're not in contact with the
object, which is what matters, and to argue someone isn't in contact with armour they're wearing is... well, absurd. It's be like saying Heat Metal shouldn't work on a sword, because the handle is wrapped in leather.
An unmanufactured metal object... plenty of metals can be found in their pure form naturally. Gold nuggets, for instance. Metal meteorites.
A castle would not be an object.
For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.
No size limit is assigned to Heat Metal, so you might be able to heat one of the walls if your GM is feeling generous.