Thank you for answering.
I am not 100% sure but think itīs Ultracote they used 30 years ago.
Have tried a heat gun and worked slowly, got most of the bigger wrinkles away
with many smaller still "open",. It seems like the glue is not actvated so cosequently
the Ultracote does not stick to the balsa. Wish I could take it off and re-glue & shrink.
Or I just might accept this old model as is.

Merry Christmas
//Rob