Very nice. It looks great. I very much like paint in a fluted seat post.

But there's an easier way. The one time I painted a fluted seat post like that, I didn't bother masking anything. I painted the flutes freehand, being reasonably careful, waited it for it to dry, then used a single-sided razor blade to scrape off anything that wasn't in the flute. Lay the blade across both sides of the flute and scrape gently. That gives a nice, crisp line, and it's then easy to scrape off any other overage. An X-acto knife would work just as well. Heck, a pocket knife would probably work just as well. A credit card might work just as well, Mine came out rather nicely; it certainly passed the five-foot test and probably the one foot test. (I can't check because I no longer have the seat post.)

This technique has limited applicability. It is 100% not recommended for cleaning up pinstriping on a frame. But it works great on a seat post.