How can i repair a nail head size leak on a vertical cast iron sewer pipe in my basement
Epoxy is unlikely to make a lasting repair. What you need is a pipe repair clamp, which is a stainless steel band with a rubber gasket inside it - the "in place repair" type have a beveled overlapping edge so the clamp can be opened up and slipped onto the pipe, then clamped in place to seal the leak. They look similar to the "hubless coupling" clamps but those typically use a gasket that needs to have the pipe cut to slip onto it.