Wood moves and if you don't allow for it you are saking for trouble. A flat bottom is a no-
no. The piece should sit on a foot which is distinct and the rest of the bottom should be
concave so as to clear the table or whatever.

Now, that will almost get you there. Even if the wood is perfectly dry when you finish the
bottom, because you are removing wood which may relieve stresses in the wood which is left,
you need to take one additional step. Make yourself a sanding block maybe 6" x 6" and glue
some 220 or so to it. After the piece has aired for a little while (maybe the next day) rub
the bottom on the sanding block to re-flatten it.

Then if it moves, you have done all that you can do...

Bill

In article ,
says...
Keith Young wrote:
I,ve been turning bowls now for a few years off and on. One thing that
bothers me is that every now and then after i finish one i notice that i
didn,t turn the bottom flat ( if that was my purpose.) Its only after the
finish is applied that this becomes obvious.Any suggestions.

If you mean that you FORGOT to turn the bottom before you finished the
bowl, I would suggest laying off the booze until you have finished in
the workshop

If you mean that what appeared flat when it was on the lathe, but looks
rippled when it is polished, then I can sympathise with that. I recently
turned a very shallow artsy piece which is basically a platter. I
thought it was flat across the 'platter' surface, but a few coats of
danish and a good buffing prooved otherwise. Flat surfaces are tricky.

More care and attention is needed. Maybe offer up a straight edge. Maybe
polish it with fine abrasive and shine a light across the 'flat'.

I am guessing though that your problem is the base that the bowl sits
on. I think the best solution is to dish it out somewhat, or decorate it
by some means. I don't find totally flat bases aesthetically pleasing.