Doing a single stripe isn't too bad. What I have tried and have been frustrated with is Monokoting the entire tube in order to avoid trying to paint during winter. Now, winter builds fly naked until the following year when I can paint during the week or two of summer where I have time.
It wasn't designed to apply on paper tubes. When heating it to activate the glue, it shrinks. This makes it difficult to make a nice even coating all the way around a large tube. I usually end up with wrinkles by the time I am 3/4 of the way around the tube and then start over, and start over, and start over again until I give up and fly it naked or fly it with wrinkles. Maybe if I had more space, a proper setup, or actual skills, it wouldn't be so bad.
The Trim Monokote is self-adhesive and putting stripes on with that is easy. Regular peel and stick vinyl works well as well. However, heat applied Monokote... yeach..
Monokote works very well on paper tubes. Sounds like you are having a technique problem.
Continually ironing it down to the tube as you work around the tube does not work well.
Best practice is to get a long straight edge of Monokote ironed neatly the the length of the tube. Tack the Monokoat to the middle of the seam to the tube, then pull one end of the seam tight and tack down that half of the seam. Then repeat this for the other half of the initial seam.
Then, wrap the MonoKote around the tube all the way back to the initial seam. It is very important to keep pulling the covering as tight as possible initially so that any wrinkles are easy to shrink out later. Leave a couple of inches extra for getting a good grip later. Pull the covering tight in the middle of the tube and tack the covering tight on top of the original seam.
Next, keep pulling the covering as tight as possible as you work the iron along the seam from the middle tacked down area out to one end of the tube. Then work the other half of the seam the same way. Working from the center of the seam out to the ends of the tube is crucial to getting a smooth covering job. This forces any wrinkles away from the center and towards the ends of the tubes.
You end up with a nearly wrinkle free tube where the covering is only attached to the seam. Pull the covering tightly over the edge of the tube and tack the covering down to the last 1/4-1/2" of the tube.
I typically trim the seam at this point using a long strip of poster board slipped under the unattached overlap to protect the Monokote on the tube, a long straight edge and a fresh number 11 blade. Then I iron down the trimmed seam tightly.
Then you have two options. Use a heat gun to shrink the covering and the lightly iron the covering to the entire body tube. If you don't have a heat gun, you can gently float the iron lightly over the covering to shrink and then iron it down more firmly.
I typically cover the ply fins first, then the tube area between the fins, then the major part of the tube do that all the seams on the tube overlap downwind.
I have Monokoted 4-5 rockets over the years. (and hundreds of model aircraft) Still have several of them. The ones from nearly 20 years back look better than my lacquer or enamel painted rockets from the same time period.
As in many model building practices, technique is everything. And with Monokote, getting everything covered tightly by the edges and manually working out wrinkles with tension
before trying to shrink and stick the rest of the covering is the key to makng it work well.
