Oops!! Don't glue or sand anything just yet. Is the leak occuring at a threaded connection or a glued connection? Sounds like it's leaking at the threaded connection.

If that's the case, 3 or 4 wraps of teflon tape will probably stop it. The entire threaded part of the pipe should be covered. (In other words, you should still see tape after you've screwed it down snugly) I think some folks might even use more tape but I normally stop at two, sometimes three wraps.