If you add the external stems now you can fair them in as you sand the hull.

Regarding sanding concaves, I use the edge of the ROS, pulling it in long even sweeps. Don't stop in one spot, and don't tilt the sander more than necessary. You will get the feel of it.

You can always finish off by hand with sandpaper wrapped around some minicell foam, but it find the ROS does the job.

Et