leaky cast iron joints

I have three cast iron connections that have moisture and a tary substance at the joint. I dont khow much about cast iron piping other than to replace it with plastic whenever possible. But this would require re-plumbing the whole drain system in the house. What is the best way and materials to use to re-seal or re-do these joints. It is 4" pipe with lead over the packing if that matters.

This is just a guess, so take it with a grain of salt. Cast iron is a very long term product. If someone tried to repair a leaking joint with tar at one point, it may be that the pipe itself is degraded enough so that it is not just the joint, but the pipe as well. Might be time to bite the bullet and replace. Depending on the water, the age, and chemicals used in the system, it could be paper thin. Hope I'm wrong, as it can be a lot of work.

cast iron pipe

if it is barely leaking,

just take a small cold chisel and haer and try tappping the
lead into the joint a little tighter.....

just go all around the joint
sometimes it works real well

you can see how it was originally done from the
way the puumber did it the first time...

also some water proof silicone will seal those joints pretty well

cast iron

It also could just need to be corked again with a corking iron.

That means you drive the lead into the joint and tighten the oakum or hemp inside that is also soaked in tar.

A corking iron is sort of like a curved chisel with one flat edge. There is one for inside the lead and another for the outside of the lead joint.

The tar is probably the tar that came on the cast iron originally and on the oakum and that doesn't mean that the pipe is bad.