My HVAC unit is up in the attic. It works fine - I just cleaned the
coils - but at each end, where the unit connects to the main intake and
output ducts, it's wrapped with this plastery-looking material. It
looks like strips of cloth that have been soaked in Plaster of Paris,
or maybe wheat paste. Some of it has cracked and is leaking air, so I
want to replace or add another layer to it. Can somebody tell me what
this stuff is? Hopefully it's something I can buy cheaply at Home
Depot.

Thanks,
Ron M.