The best way to put this answer is think of your 'Skin' as armor...
That being said, compare your skin armor and your material armor; the higher of the two types, in any of the elemental resistances, wins the right to be checked for actual resistance VS. elemental attack.
Your natural resistances never go away, and do not stack with any other resistance type {that I am aware of}. They simply exist underneath it all.
--Sturger--