Pictures would have helped to know what pattern you are using but,
I surmise, it would have been much easier to sand it BEFORE the bottom
and BEFORE the handle were attached.
Steve Good has a pattern for a sanding mop that you could make with an
extra long shaft to get inside the stein but, I think a lot of hand sanding may
be necessary since you say it is already assembled.
God Bless! Spirithorse