Hello Paul, very nice and well done.
There is no need to power sand prior to gluing up, mount it on your lathe and turn it flat. I have abrasive fixed to a bit of (planed up) 2x2 which I carefully hold against the revolving disc, as long as you are not trying to take 1/4 in off at a time this will give you a true enough surface for gluing. Wear a face mask and use extraction though as it does create a bit of dust.
Hope this helps ,
Regards
John BHT