How to seal chimney crown?

What should I use to seal my chimney crown? It is 30 yo and looks to be made of morter!! I've used Thompson's this year and silicone around clay tile, When it rains hard it drips on my insert and rust's it.... Aaron

Sounds like it's going down the flue. If it's coming in the crown and getting to the insert you have major problems as there would have to be breech in your flue tiles. Probably a silly question but do you have a chimney cap? Is the insert lined? If so how is the liner terminated?

yes its has a full 6" flex. s.s. liner, Its not the flashing or the rain collar or cap but it does follow my liner right down the back side? Top flashing is siliconed? I'm just looking for a product to seal my crown to get me through the winter.

If the cracks aren't too large you can use a product called crown coat made by Homesaver it works good. Crownsaver is good for worse damage. Both products are easy to apply set up quickly and offer a permanent fix. If it's following your liner that's the area I would look at. Could it be condensation? I ask because I'm not sure exactly how much water were talking about if your liners insulated etc. I've seen silicone separate from the metal on top plates etc. and maintain a sealed look as well.

Heat should not be that much a factor. Silicone should work but movement from contraction and expansion or ever high wind can break that seal I suggest you remove the existing caulk and re apply a decent brand 35 year plus flexible caulk are you sure it is an outside the liner leak ? have you ruled out condensation is the liner insulated. While at it caulk the existing clay liner as well. Chizsel out all cap cracks and use drylock type cement to seal them