I'm a first-timer and I've installed a new sink, toilet, and tub in my one and only bathroom. Had issues with each as a newby, but all in now with no leaks except the sink drainpipe. In the wall is cast drain/vent and I am inserting a plastic pipe from p-trap ... I've tried to clean the inside of the iron and even put some sealant around the plastic but it's still leaking at the nut when I test. How can I seal this without having to replace the cast-iron in the wall?