I have an older house and the main drain that goes to the floor has a coupler between the 3 Inch PVC to the Old Cast Iron Pipe. It has started leaking a slight leak when running any water. Is there a good way to fix this? Should I emery cloth the cast iron side to remove anything there and get a new coupler and replace the coupler? Or is there something I can apply to the cast iron to better seal it? After sanding down the cast iron and getting a new coupler? Any suggestions will be welcome?