New sink drainpipe leak (plastic to cast)

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?

What kind of washer are you using between the steel pipe and plastic pipe? Is it a nylon washer with beveled edge or like a very thick rubber band? You should have a galvanized pipe (nipple) coming out of the cast iron to which you insert the plastic with some type of washer and nut around the plastic. I've seen all kinds of washers used but usually it's the rubber band looking washer and frequently people wrap it with a packing type of string (can't remember the correct name here) if it leaks. The nylon washers work pretty good but IMHO the others are better in this situation. Be sure the nut is not cross threaded on the pipe nipple.

I guess I don't understand what your saying. But if you are saying that you are trying to screw an 1 1/2" pvc adapter into the 1 1'2' TAPPING in the cast iron tee and it's leaking, take it out and run an 1 1/2" tap over the threads and screw the PVC adapter back in with some Teflon pipe dope. If the cast iron is in bad condition, use some Teflon tape before using the compound.

clarification

Sorry I wasn't clear ... no PVC in this joint that I know of ... I think it's ABS that the P-trap is made of ... anyway the p-trap set came with the nuts and wedge nylon washers, but in looking at it now, the receiving "nipple" is not tapered on the inside, so maybe that is the problem. plumbing was old and a bit of a cobble-job, so my fear is that they did something unorthodox, but the nut threads on (not cross-threaded) and can be snugged, but it still leaks ... I'll try a rubber washer, but don't know how that will snug around the ABS like the nylon would?

Can you tell which side of the nut it is leaking on? If it is leaking on the trap side then it's definitely the washer. If it's leaking on the nipple side then it could be the pipe threads which you "might" get it to stop leaking with teflon tape or pipe joint compound (TFE is best when using plastic). Sometimes I just get frustrated with those kinds of problems, wrap the heck out of it with kite string or some type of heavy cotton string, coat it with pipe dope, force the nut on and snug it down... then close my eyes and hope for the best. Then if I'm really ready to give up I just grab the duct tape.