I had a leak where the exit fitting spin weld partially failed on my black tank. My fix was to go to the hardware store and buy a can of ABS joint cement and a small package of fiber glass fabric. After cleaning the area with acetone on a rag, coat the area with joint cement and while still wet, embed the fiber glass fabric. Allow to dry, repeat at least two more times, then overcoat with the ABS joint cement. I found, by research, that the joint cement is nothing more than ABS plastic dissolved in acetone so the repair is actually adding, not only additional ABS, but adding fiber glass reinforcement to the area.