You can shrink cotton with BOILING HOT water. When you pour it on slowly, the canvas will shrink so much that you can hear the frame creaks. When it dries, it will relax a little bit but tighter than before you start.
The key is the high temperature -- sort of like your cotton T shirt will not shrink until you tose it into the dryer. The problem you face is failure to pre shrink before oil base coating. I have not used any oil base before, and should not command on that. I have bought a can of snow roof as recommand by Mike. Hopeful I'll be done before summer end, so I can use the yak this year.
Good luck on your project.

: I an in the process of building a stick frame kayak using "Wood and
: Canvas Kayak Building" written by George Putz. I have the frame done
: and have tried to put #12 canvas on. I must have done it wrong. When I put
: the canvas on it was tight but after I put my first coat of oil based
: polyurethane paint on the skin and when it dried it was all wrinkled. I
: probabaly did not strecth it enough. Could I have some how shrunk the
: canvas before I painted to keep it tight.
: I have removed the canvas and would like to know about using nylon. In
: WoodenBoat there was and article on building a baidarka kayak using woven
: nylon cloth and epoxy-urethane sealant. In reading the threads I get
: confused. Can you heat shrink nylon. I bought a yard of flag grade nylon
: and heated it. It shrank but returned to original size after it cooled.
: The book suggests Dacron but I have heard epoxy does not bond good to
: dacron. I would appreciate any help I can get. Thank you/ Ken Moran