That's a great looking blank - very unusual effect, and I like what you have done with it! Looks very good.
I have played around with trying to smooth out bores on metals - and I think that the principles move from one material to another. It's the same process as the one you have applied to get that smooth, shiny exterior. First you cut as smoothly as you can - well-sharpened drill, then a reamer if you have one (you can get expanding reamers that cover a range of diameters). After that I start with sanding as thewishman suggests - just like the exterior I use finer and finer grades through to wet and dry 1200 / 1500 paper around / through a bar of some sort. To get smoother than that I have used lapping pastes- you can get very, very fine pastes equivalent to 20,000 grit. The tool to use needs to be softer than the material you are lapping - to lap steel I use brass. The paste enbeds in the brass and wears away the steel. For plastics I think I would try wood - and finish up with metal polish using a cloth on a small bar.
Hopefully someone with more experience / knowledge will respond and we'll both learn something!
Good luck and please let us know how you get on.