From what I can understand of publish 94 only a focused necromancer should be able to lower a players resist caps with corpse skin. Unfocused necromancers casting corpse skin should only lower the resists of the target by 15. Allowing the ability of making CS proof suits for unfocused necros by having 85 fire/poison.
In testing my unfocused necro/mage/sw/Eval character can lower the cap of a player to 55 fire/poison.
uo.com
Necromancy Update:
Focused Spec Necromancers when casting Corpse Skin will reduced max poison and fire resist based on the casters real necromancy and spirit speak skill.
www.uoguide.com
Has anyone got a method of testing this on OSI?
In testing my unfocused necro/mage/sw/Eval character can lower the cap of a player to 55 fire/poison.
Publish 94 – Ultima Online

Focused Spec Necromancers when casting Corpse Skin will reduced max poison and fire resist based on the casters real necromancy and spirit speak skill.