Hi David,
i had the same problem. I read about one chap who uses cotton buds in a drill with different papers then liquid abrasive. It didn't work well for me though.
I think i read somewhere the secret is maybe steadily increasing the size of the drill bit, (more regularly than the pilot hole and final size i did), lubricated and slowly, Then repeat the above.
It makes sense if you have a smoother inside surface to start with.
I haven't had the chance to try it myself yet as i have been completely sidetracked.
If i can find it again i will pm the link.
I am going to have another go as soon as i get a chance, for what its worth i think they are great.
There are some really expensive ones as well Andy's Pens - Onoto 2012 Magna Plunger-Filler Limited Edition
Yours is better than my crap first attempt.
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Regards
Pete