There's still a tiny bit of clay/rust compound stuck to the edges around the pipe. I used a wire wheel on a drill, a razor, and god knows what else. I'll try a file next.
You need to clean around the entire outer surface of the cast iron pipe to the depth of insertion into the no-hub. Considering how shallow the trench looks under the pipe (perhaps it's just hard to see from the photo), it appears you didn't get a wire wheel under the pipe to clean the bottom surface. The pipe should be a smooth as possible. One way to do this without a lot of clearance under the pipe is to get a long strip of coarse emery cloth, slide it under the pipe, and work it back and forth to smooth the pipe. This is easier than a file and you don't risk putting flat spots or gouges in the pipe.
After you mechanically clean the pipe really well, wipe it off with a rag soaked in mineral spirits and let it dry. Then apply
Black Swan No-Hub Sealant to the outside of the pipe and then slide your no-hub onto the pipe (apply the sealant to the cast iron pipe ONLY; do not apply to the ABS side). The no-hub sealant will fill any leak paths in the surface of the cast iron pipe caused by rust or other damage. This stuff is a bit expensive, but it will ensure a no-leak seal. No one wants sewage leaking in or around their house.
After you have sealed and tested your joint to be leak-free, then you can apply a seal coat to the outside of the cast iron pipe to stop rust around the area of your repair.
I would not recommend putting any silicone plumber's grease on the pipe. Doing so will prevent you from applying any sealant to the pipe, either inside the no-hub, or on the outside of the pipe to prevent rust.