[DanEC]

There is an exhaust and muffler sealer available. It used to be sold as a Walker product but the last tube I bought from NAPA was no longer labeled as a Walker product.

This stuff seals up well but it does set pretty hard and if there is any movement at the slip connection as the car is driven, it may eventually leak again - but probably not as much. It doesn't seem to hinder disassembly later - I just spray a little WD-40 in the connection and that seems to lubricate and soften it enough that the pipes will come apart without a big struggle.

Edit: Kevin beat me to the punch.