I've always used a brush instead of crocus cloth. Any reason i should try the grit instead? Does the cloth leave a smoother or cleaner surface for soldering or compression ferrules?

Here's the problem I'm having. I was clearing a clog in the wall coupler (not sure this is the right name for the part, I'm talking about the piece that goes from the p-trap to the cast iron nipple sticking out of the wall) underneath the kitchen sink in my rental unit. It does not look like the joint with the nipple was sealed and the threads are very corroded, so when I removed the wall coupler a small piece of the end of the nipple chipped off. This is what it looks like... you can see the chip along the top edge, it's about 1/4" deep. Now when I reconnect everything, there's a small leak at the joint with the nipple. It doesn't seem like pipe joint compound would do much; currently thinking plumber's tape or epoxy, or a rubber coupler or something, but not sure if or which of any of those would do the trick. Any advice on stopping the leak? Seems like the proper fix would be cutting the pipe and some new threads, then buying a slightly longer coupler and properly sealing the joint, but I don't want to do all that if there's a simpler (if jankier) fix. A trap adapter was suggested on another forum. Thanks in advance.