Dismantled a botched PVC-to-cast iron joint that had a 3" PVC pipe inserted about 8" into a 4" cast iron pipe (not hub, but the pipe itself), and then a Fernco rubber boot over the whole kit'n'kaboodle. The end of the cast iron pipe is pretty badly corroded in places, so much so that I could crumble off bits of it. This "handywork" dates back to 1994, so it didn't take terribly long for this state of affairs to develop.

Now, I suspect that it was the fact that there was a place for stagnant water to collect (between the PVC pipe and the cast iron) which eventually caused crevice corrosion; I've seen stainless steel crumble into nothing in a small area under similar circumstances (under a gasketed cover) with the rest of it being pristine. With a code-compliant reinforced coupling this issue may not exist, but I'm still a little leery - should I or can I apply sealant between the inner lip of the coupling and the edge of the cast iron to ensure that no water will be able to collect in that space, or between the rubber and the cast iron?