I once used Helmsman spray on as a finish.
The pen I was making used purpleheart for a center band, and I wanted something with a UV coating. I started with Deft Clear gloss lacquer spray, then followed up with three coats of Helmsman.
I didn't use any polishing or sanding after that, and was happy with the result. It wasn't a high gloss like CA polished to 12000 grit, but it was very nice IMHO and showed off the chatoyance well.
If you do use polyurethane, remember that most of it is amber in tint. Helmsman is clear that I know of, and don't know about the others. That's why I used Lacquer on my other pen.
Sorry about the bad pics. This was before reading anything about taking them, and I'm still learning.