Duct Tape, or what?

I am trying to seal my old hot air heating/air conditioning ducts. I was told not to use the traditional duct tape, but to use special tape and mastic. My ducts are the flexible type, with a thin layer of insulation.
I found several kinds of metal tape at the HD. Are they all suitable for heating systems? The assistant did not have a clue.
I did not find any mastic at the HD. Any recommendation for the mastic and where to buy it?
Thanks
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Walter
www.rationality.net
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My HD does have the "mastic" in small expensive cans.
I use the metal duct tape, lasts for ever.
You will need to pull back the insulation and seal the flex (usually plastic) then pull over the insulation and I use a ty-wrap then tape.
And you really thought the assistants at the borg stores had a clue?
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Use aluminum tape 1-2 mil thick that is the best thing I have found to seal ducts. Duct tape only sticks to itself over time. The other thing I used two houses ago was butyl <sp> caulk it doesn't dry it stretches and gives with out giving up.
Rich
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