Kevin
I have three basic ways to finish off a bowl bottom.
1) jam chucking
2) working the bottom against a sanding pad held in the lathe
3) using home-made large jaws (see the web site)
in the past I have also used a compression or dough-nut chuck.

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God bless and safe turning
Darrell Feltmate
Truro, NS Canada
www.aroundthewoods.com