First let me say that I love the whole SOF concept, feel very satisfied with the kit and support I got from Jeff, and in no way am I maligning Kudzu.
If memory serves, I had to get the 9 oz fabric because the Castaway design is too beam-y for the premium, lighter fabric. I would gladly have paid the premium if the lighter fabric would have been wide enough for my boat.
I sewed it a tight as possible, starting over several times to get it right. In fact, I sewed the center seam around 3/16" fiberglass rods rather than using paracord for the corded stitch. I tried it with paracord and couldn't get a decently straight line. The fiberglass rods gave me real leverage to allow me to get it super tight and gave me a straight seam. I pulled the artificial sinew to breaking point multiple times between each end and the cockpit. Not fun starting the next stitches. Do not copy this process unless you have the persistence of a hungry puppy - it's a major pain in the butt, and will lead to much frustration, yelling, and occasional swearing.
I ran an experiment on shrinking the 9 oz fabric. Marked a 6" + on a scrap, then ironed and steamed it until there was no more movement. The most I got was about 5/8" over the 6 inches of the +, and that only in one direction. The other dimension was less the 1/4" of shrinkage. So, I knew I couldn't rely on shrinking to get it tight. It was TIGHT before I ironed it.
So, it's like I said, it's not a piano, it's a boat that I intend to use daily - the photo below is where I get to live here in South Florida. I plan on using that Castaway daily to paddle and fish, wrinkles be damned.
My only point in raising this whole wrinkle issues is to alert the community that there may be a more generalized problem than just Zar on the 9 oz fabric, one that extends to the most trusted and recommended of finishes for these boats, Rustoleum.