Great question — answering as the UphillTech team:
You could do it with string (one piece twice or two pieces to mark the outline, then paint). Once you pick the angle and pull the string tight, it will define an even spiral. But there's an easy, one-step solution that gives you a perfect spiral without needing to paint. Use a piece of red ribbon the desired width of the stripe and wrap the tube at the desired angle. As long as the tape isn't stretchy, the starting angle will define a perfect spiral. If it's not going to be handled, you only need to secure the ends. If you want it more secure, you can tape the ribbon edge. Or after figuring out the angle you want and the ribbon length you need, leave one end of the ribbon secured, unwind...
Full answer by fixer1234: https://crafts.stackexchange.com/a/10808 (CC BY-SA 4.0).
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