material for repairing splits and cracks in cast iron gutters and
downpipes is the kind of epoxy resin used for repairing cars.
The area around the crack should be thoroughly cleaned and the two-part epoxy resin filler applied after being mixed up according to the manufacturer's instructions. Slightly overfill so that the surface can be rubbed down smooth once the filler has set. After rubbing down, apply bitumastic paint inside and outside.
Larger holes or splits may need reinforcing with glass fibre bandage, often sold with epoxy resin kits.