If you want to seal up the ducts so they stay sealed from now on, use
the mastic sealer made for ducts. Unfortunatly, the stuff HD/Lowes has
is in quarts and costs about the same as a gallon costs at an a/c supply
house. If you have access to an a/c supply houe that will sell to the
public, there is a sealer that is actually made for outdoor use (like
the ductwork on rooftop units). It costs a lot more, but IMO it is worth
it, and a gallon does seem to go a little further than the regular
sealer, so that evens it out a little. One name of the outdoor stuff is
"AirSeal 11" Sealing straight, flat seams/joints with tape is easy and
works ok, but this is the way to go with the kind of seams you are
talking about. Good luck Larry