As discussed in several of the questions & answers, I am trying to use a neoprene coupling to connect an older cast iron 4" pipe to a replacement section of PVC - below grade.
I have cut back the cast iron pipe a couple of times; the pipe is sound but the outside surface is not smooth enough to get a good seal with the neoprene coupling. I have tried to clean off and smooth the surface but I still get some leakage when the coupling is connected. Is there a chalking or epoxy compound that I can use on the cast iron pipe before connecting the neoprene coupling?

I would try something like 3M's 5200(Marine Caulk). Smooth around pipe, let dry or not, then reassemble.
Maybe use Ospho(Osphoric Acid) to convert Rust first.