A trick that I stumbled on when I ran out of tauting dope and was too impatient to wait on an order using polyspan was to dope it with the non-tauting then hit it with a heat gun while it was still plastic.  After two coats like this it was drum tight and has stayed that way for a year.  The good (and bad) thing about polyspan is that it will keep shrinking if you keep heating it - right up to the point it melts.  Once those tauting coats dried, the non-tauting did not loosen it.

Ken