Do I need to seal lengthwise ductwork joints?

[tedrussell]

That stuff works really well, but the OP should be advised that it will make a mess.  If you want it to look neat, use masking tape on both sides of the joint, then brush it on.  It's really hard to clean off if you slop it onto places you don't want it.  If you don't care, like if putting the seams on top or behind where you can't see them, then it doesn't matter.  I sealed the joints on my cyclone when I built it, thinking it would just wipe off excess like caulk, but noooooooo, it didn't.  

Thanks for the excellent advice.  How do I apply it?

[R Clark]

The HVAC guys who did our new heat pump/propane burner install last year used it.  They brushed it on.  Those guys were way neater than I would have been with it.  I always seem to make a mess with that kind of thing.

[TDKPE]

Thanks for the excellent advice.  How do I apply it?

Like WxMan said, I used a little brush.  Probably one of those little stiff ones with metal shaft.