I'm not exactly sure where your leak is, but it sounds like a flue pipe. If your draft is good, there should be no leaks at any joints. So, I would look at the draft issue first.

Yeah like Bay says Loger . Unless you have improper Back Pressure within your Flue (Damper closed effect) you should not have any smoking joints at least not when the flue pipes are up to temp . You should never have any piping restrictions down stream of your Damper . This would include above your ridge strap joint . All sealants I have ever witnessed dry out and leak , thus the reason of proper flue installation and connector straps or bands. I have seen when Selkirk Stainless joints become old some friends and I at our Hunting Camps have inserted Graphonic Rope Packing to work as a gasket effect between the bell and spicket (interlocking) ridge . This product is available at any Industrial or Commercial Pump or Valve Distributor . This product is very pliable does not dry out conforms to irregular surfaces and is good for temps over 1200 F . Check out what Bay & I have advised and then get some 3/16" or 1/4 " Graphonic Packing tamp it down to thickness you wish and insert within the joints and you will be good to go for the life of your stove Bro !

I must admit my almost closed damper and modifying my fireplace are my âÂÂRealâ problems.

1. Related to damper, I keep it almost closed in my 14â ID 16âÂÂOD insulated filled pipes (2 X 30â sections) to try and hold more heat.
2. My fireplace was opened to both sides requiring the large pipe. I added a 14 or 16 gauge steel back to one side, stainless grates below and above the fire, a plate on the upper grate 7 X 2â tubes to slow the heat at the tubes.
3. The approx 30 year old project will expand and seep a little where the upper grate is bolted in vs welded (more bolts might help). âÂÂBurning hot fires and choking the draft is the real problemâÂÂ, but sealer yearly (in the few 1/6â buckles) has solved it until it flakes off. IâÂÂll have to open the damper more. Plus, ext air is ducked down into the hearth in front behind glass doors to allow overnight burning.

Would you call it a 30-year-old liability or just open the damper more and keep sealing? IâÂÂll post pics if you are interest in seeing the project. loger

http://i1019.photobucket.com/albums/af312/JOW_06/fireplace2insert.jpg?t=1290271118

http://i1019.photobucket.com/albums/af312/JOW_06/fireplace2insert.jpg?t=1290271321

http://i1019.photobucket.com/albums/af312/JOW_06/fireplacesinsertbacksidesignsofredhotheat.jpg?

It's a bit complicated to solve here.