What are the proper ways to connect PVC to galvanized metal?
The context of this question is for a house that has a kitchen drain stack made of 2" PVC, except for the topmost (vent) section, which is 1.5" galvanized metal. The metal pipe is connected to the PVC stack by one of those flexible couplings made by Fernco.
If there is too much water in the sink draining at once, water will actually go up the vent portion and leak out of the coupling at the top. It leaks with just enough force to squirt water up about a half inch.
The metal pipe is of course corroding on the inside, but not bad enough to cause major blockage or anything.
Is there a better way to for this to be connected? I don't know how to transition from PVC to galvanized metal.
I tried tightening the clamps on the coupling, but it didn't help. I'm worried that tightening it too much could deform the pipe and make things worse. I also tried seating the coupling slightly higher up... still no better.
I found a video of someone using a thread sealant between a galvanized pipe and one of these flexible couplings... could that really help? Or perhaps make things worse? They used Blue Monster sealant: