I begin to sand the inside using 60 grit on the drill pad and with the lathe stopped. The motor on this one is so heavy there is no need to use the spindle lock.
Woodturning an Incurved Winged Bowl - Page 8
For work like this it is often best to stop the wood lathe and sand with a power pad.
Even with the dust collector going you need a good sanding mask here and every so often, blow off the surface. You can see from the dust that this is a cutting operation as much as a smoothing.
You can see here the need for 60 grit.
Here is the result of careful work, even at 60.
Proceed through the grits using each one to remove the scratches of the previous.
If sanding at a fine grit reveals scratches like this, back up a couple of grits and redo that section. Do not try to sand out the natural coloration of the wood. It has been done. We call the results holes.
Once sanding is finished to your satisfaction, remove the piece from the lathe. Now we can think about the bottom.