Ok I need to remove the vent for my furnace to remove the skin. All of the screws are taken out and the seal with the butyl tape has been loosened. I don't really know anything about theses old furnaces but how do I get the outside vent/flange off in order to take off the skin? Photo of vent below
I think you could clean the tubes very well to fit them back together. These can be a real bear... try not to bend things too much. It helps a lot to remove whatever is above the furnace for access. Good luck and let us know how it goes.