The game has been programed so the the Armour is destroyed when you leave the underdark and not when it is exposed to sunlight as it is supposed to do, So I suppose that you could use shadow keeper to import the armour onto your character before you enter the underdark and once again after you leave the underdark when the armour is destroyed.
P.S If you dont know what shadow keeper is it is a saved game editor that you can grt from:
http://www.mud-master.com/shadowkeeper/