I originally posted this in HVAC, but was told that a plumber might be best to advise on this one.

I have discovered a leak in an 80 year old steam radiator system. The pipe shown is 4" diameter cast iron that holds steam. It is below the floor and there is dirt below the pipe. It leaks at the seams where the old is attached to the new.

It looks to me like someone cut out a section of the cast iron and attempted to patch this a few years and did not do a good job. If I dig around this and pack a ton of bondo around the seams, will that solve the steam leak?

If not and I have to cut the cast iron, how do I attach the middle section to the old pipe? Do you weld it? If I have to weld it, how do I weld beneath the pipe?

Thanks for the help.

Try using a no hub band. I actually do not know if this will work, I have not worked on steam lines.
I found this by doing a search on steam heating Radiant, Baseboard and Steam Heat - HeatingHelp.com