I have been working on my first shamisen in sapele. I just finished the sao and learned some things along the way I would like to share.
For making an Ipponsao it is necessary to run lines along the whole length with a marking gauge. This ensures a consistent plane along the length. Gluing the sao profile on the end and then sending lines down with the marking gauge will mark the material to be removed. Register against the flat front face for the length of the sao and against the curved backside for the bottom. A line marked on the left should be marked on the right too - this establishes good symmetry.
A plane is not very effective for removing most of the material; A drawknife and rasp are best for the back. Since this sapele is figured, it was cleaned up with card scrapers.
A relatively thin card scraper works as a great way to remove tear out and round corners. Bending it around the existing shape will “average” out the surface. Be cautious though, a card scraper is not a plane and will make the surface uneven. It should be used in long strokes and sparingly to preserve the consistency of the form.
Finally, one last thing to mention is to check the quality of your work with a high gloss finish. I do a french polish with shellac. When you have a reflective surface on the piece and you sight down the sao’s length the flaws in the surface will become apparent.
I think this procedure was very effective for shaping the sao and I think you will see this in the result (ignore the mess from renovation). Next will be the dou and when finished I will follow up with my experience doing that.