It adds the bonus to exceptional resists to all crafted armor (other than shields) that doesn't have pre-defined values, regardless if made of metal, leather or wood (I don't play gargoyles, so I've never checked stone armor - and I've not made any of the masks since they became craftable, either).
So, the +5 additional random resists would be added to the 15 random bonus resists for things like
Helmets
leather, studded and bone armor
chain, ring and plate armor
Leaf, hide, and wood armor
What it doesn't add to:
Non-exceptional armor of any type
Shields
The 3 Tailor recipe items (Song woven mantle, Stitcher's Mittens, spell-woven britches)
The Special wood armor types (Bramble Coat, Darkwood Armor, etc.)
So, it should be on any crafter that makes armor or weapons (though you can soulstone the skill and pass it back and forth between characters, so that you don't have to train it more than once, if they are all on the same account on the same shard)