I have an old home built in the 1920's with a cast iron vent stack pipe running across the floor of the attic tying from plumbing on one side of the house to the vertical stack on the other side. Part of the underside of the cast iron has corroded to the point that moisture is leaking from the pipe and onto the ceiling of the room below. Is there a sealer I can apply to the holes in the pipe or do I need to cut the section out and replace with PVC? The bottom of the pipe is not easily accessible.