I am turning a bowl (about 6" in diameter by about 6" deep, side grain) from
green Black Walnut. I have shaped the exterior of the bowl. When I started
to power sand, I noticed that the outside of the bowl had several concentric
ripples. These ripples are a bit too deep to remove by power sanding, but
have eluded removal with a bowl gouge or scraper. I have not hollowed the
bowl yet, so it shouldn't be moving too much.

I have tried taking a very fine cut with a sharp bowl gouge, but the new
surface had just as many ripples. What is going on here? I'm pretty new to
green wood turning. Do I just need to rough the bowl and let it dry for a
couple of weeks before I proceed? Any suggestions will be much appreciated.
Thanks.

Barry