Marc, not sure what you mean by a "smoother surface". I fill voids all the time, and much prefer crushed to powdered stone. Powdered stone, to me, looks fake, like powdered plastic or something. Crushed gives me, and I believe my customers, the feeling that an actual stone or shell product is in there. The only places I ever use powdered anything is if the void is too narrow, which I'm doing right now on a peppermill. Just my preference, I'm probably not getting what you're saying. I finish with a coating of thick CA, so it's every bit as smooth as the surrounding suface.
And Alice, not only keep trying, but LOOK for woods with voids in them! In my opinion, they're gold, unless of course they're to be used for food service, then they're useless. Otherwise, real artistic possibilities come in when you get LOTS of voids. I'm even making voids on purpose now, a la Stephen Hatcher, where I carve a scene into the bowl or platter and fill with crushed stone or shell. I have nothing to show for it because it seems to take half a lifetime to learn the cutting/carving part, but I WILL get there! As far as a nice smooth transition on the bottom, Marc's right, get the heaviest, thickest scraper you can find, learn how to sharpen and use it, and use a super light touch. Then practice, practice, pratice. Trust me, the person you gave the bowl to as a gift is already treasuring it.
Good job.
Dale
P.S. I could not live without my Sioux sander!