I have no solution for this boat, but for the next one try this. It works quite well...... After you stretch the skin on the frame, wash it down with hot water, using a sponge. That will pre-shrink it. There are some instructions for doing it on my pages.
> I am constructing my 4th wood and canvas kayak. This project
> is an 18 1/2 ft double based on Norman Skene's 1923 Walrus.

> I've never run into this problem before...but I'm sure that it
> isn't unique. I stretched the skin onto the frame 2 days ago. I was
> very pleased with the results. Yesterday I killed the weave with 2
> coats of oil based paint and talcum powder. Last night it was still
> tight and the paint was fairly hard. We had a heavy dew and now my
> 'yak sports an "elephant skin". I am sure (I think) that
> the humidity allowed the skin to stretch and suspect that it will
> shrink (somewhat) when and if the humidity drops. What can I do to
> correct this (I suspect nothing). What can I do to avoid this in the
> future? Apply canvas damp? Only canvas on foggy days?

> Thanks...I'm about ready to break her up for matchsticks!