Hello:

Regarding all the round sheet metal duct work exiting our typical gas fired,
hot air heating
furnace:

What's the best way to seal these duct joints, at least where one can
actually see them ?

Duct tape doesn't really sound like the best way to go.

Would appreciate any suggestions as to brands, etc.

I guess I should also ask:

Is it necessary to do so for the hot air ducts ?

How about for the duct leading to roof flue exhaust (no fireplace in house)
?

Thanks,
Bob