Small holes. Clean metal thoroughly with steel wool. Smear on plastic steel and trowel smooth on both sides of hole.
Large holes. Have these welded.
Seams split. Clean thoroughly with steel wool. Press together (if necessary, have someone bear down on the seam with a piece of pipe). Then heat with torch and run in solder (see Basic Methods: How to Solder Metal). Don't release tension on seam until solder has hardened.