For me, most of my bottoms are a mortice (recess) which is sanded and
that is my finished bottom. This is the easiest for me.

While mounted, you can use a parting tool to part of the mortice, then
sand out the marks. This will also work for a tenon.

You can sand the bottom, and then apply a waste block using hot melt
glue (see Darrell's site), wood glue and paper or super glue, and waste
block with mortice or tenon, and after turning the inside. The waste
block parts off easily with a chisel and there is minimal cleanup.

If you don't have a chuck, and use a face plate, you just part it off
the lathe and sand it out.

Belt sanders can be used to clean up the bottoms of bowls.

You can also make a disc sander that fits on your lathe for sanding the
bottom of your bowls.

I use the angle drills for sanding out the mortices on my bowls. I have
1, 2, and 3 inch discs, which covers all of the mortice sizes that I
use.

robo hippy