RE: Ultra RC 87" Yak Build and Fly
Sometimes it does take a few passes to get it shrunk. It's Ultracoate and IMO has the best shrink/retention of them all. Still when being covered they sometimes leave a little too much slack... shrinks up under heat but over time sags again.
Again... over time it will eventually shink up and hold well. And yes... your airframe is probably pretty "wet" right now. I did a Lanier Yak and it lost nealry 1/4 lb in water weight just sitting in the shop... no lie. THis shrnkage will indeed add to the covering sag.
Don't go overboard in shrinking it... just taught it up... and before your ready to fly... it'll get there (or should).
Where is your's loose? Mine is drum taught on all three except maybe for a bay or two under the tail where the stringers are... (these too will bend and dry letting the covering sag again... that' why I recommend you just taught it and leave it til the airframe is "cured".