I would also like to know where to find correct information on what size gasket material I need to use for the doors on this Fisher unit. I bought it second hand and do not have any manuals etc on this unit.
You have Series III doors that use flat gasket that fits in the door seal channel on stove front. The Imperial Cement you have is for that use. It is also for cementing joints on cast iron stoves. The outlet area gets too hot for cement. If there is any original cement in door seal channel, remove with wire wheel to bare metal since it makes the gasket material too thick. Any width that fits in seal channel is fine.
High temperature silicone may work longer, but is not the correct way to solve the problem at connection, and no sealant is required.
The problem is not enough heat in chimney to create the correct draft.
An 8 inch chimney diameter has more CAPACITY than 6. Almost twice as much. So the Insert was designed with an 8 inch outlet to allow enough heat up a larger existing fireplace flue. Continuing that size to the top is one way to cure the problem, but also uses much more heat up the stack than the smaller 6 inch. Reducing will not allow the full BTU output of the stove, but they are seldom used for maximum output anyway. A 6 inch pipe and chimney works in many cases, but can cause issues in installations where the chimney is too tall and cools too much, pipe configuration uses elbows that create resistance, or high single wall pipe allows excessive cooling before entering chimney. That’s why we need to know chimney height, and type.
You need to maintain 250*f. flue gas temp inside chimney flue to the top of chimney when smoke is present. This creates the draft required at stove outlet to allow air into stove. Below this critical temperature, water vapor from combustion condenses on flue walls and allows smoke particles to stick forming creosote.
Is the ceiling over 8 feet?
Do you have a magnetic pipe thermometer on the single wall pipe showing surface temperature? You need that so you know when and IF you can slow rising gasses with a flue damper.
Start with pipe damper open. Bring up to temperature, and control fire with intake dampers. The flue pipe damper is a variable resistance. It is used for control of an over drafting chimney by slowing the velocity of rising gasses. The damper should only be needed for open door burning with screen in place. If smoke exits any pipe joints there is a draft problem. The correct heat inside the chimney flue creates a low pressure area inside chimney, pipe and stove. This allows higher atmospheric air pressure to PUSH into stove intakes. Any leaks allow indoor air IN, not smoke out. The detrimental effects of a leak is cooling chimney.