i am connecting 4 inch cast iron pipe to 4 inch pvc pipe using a fernco coupler. the coupler is the correct size yet it (the coupler) fits somewhat loosely around the iron pipe (there is about a quarter inch gap between the outer surface of the pipe and inner surface of the fernco). can i use a lap/gutter rubber butyl type caulk and fill in this gap before i tighten down the metal strap on the fernco? i don’t want any water getting in there. maybe this is overdoing it a bit.
thanks for any help, marc