Cast iron is a strong, durable material used in many applications such as old bathtubs and drain pipes. Over time however, exposure to moisture as well as creeping weather elements can cause cast-iron drain pipes to rust. If the pipes are not cracked or split by the rust, you can simply scrape off the rust with a wire brush and sandpaper. But if the drain pipe is leaking, you'll have to replace that section of pipe.
Shut off the water supply to the house. This will ensure no one accidentally uses a sink, causing water to drain out of the pipes while you repair them.
Put on safety glasses. Cut out the rusted section of the cast iron drain pipe with a reciprocating saw, using a metal cutting blade. Place the metal cutting blade against the rusted pipe, about 3 inches away from the rust where the pipe is not rusted. Set the reciprocating saw to its highest setting. Turn the saw on a cut downward at a 90-degree angle.
Smooth both ends of the cast iron pipe where the rusted section was cut out with a metal file and sandpaper. Place the metal file against the cut ends of the pipe and rub it up and down and back and forth until the metal shards from the cut are gone. Smooth the pipe ends using 40-grit sandpaper, rubbing the sandpaper around the outside and inside the pipe ends.
Fit a hand pipe threader over one cut end of the cast iron pipe. Set it to the appropriate diameter and turn the handle around the pipe to make thread grooves on the ends of the cast iron pipe. Release the pipe threader and repeat on the other cut end of the pipe.
Spread pipe sealant around each end of the cut pipe using the brush attached to the pipe sealer's container. Fit pipe connectors that are larger in diameter than the replacement pipe on each cut end of the cast iron pipe. Tighten both connectors with a plumber's wrench.
Spread pipe sealant around the inside of each end of the replacement pipe using the brush attached to the lid of the sealer's container. Fit the replacement pipe between the two connectors. Tighten the connectors to the replacement pipe with a wrench.
Turn on the water supply to the house.