repairing deteriorating ac duct work

I have a home that was built in '85 that has this tubular gray ac ducting. It is covered with a thin plastic film and over the years this grey film is just falling appart causing the entire insulation to fall away from the actual underlying duct. I know the best thing would be to replace the entire duct but it would seem that I could just wrap another different more robust plastic film around the duct, tape in place and be back as good as new. I hear that this duct problem was quite common. Is there some product out there that is made to fix this kind of problem. I could envision some kind of plastic zip over sleave would do a nice job.
btw, is really my correct email address, so far its worked.
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