flattening bottom

Quoted text begins.Re: A concrete sidewalk (or driveway) works well too, with or without sand,
as long as it wasn't 'floated smooth' by the mason when it was setting.
End of quote.

If you use blue builder's chalk on the concrete first, then when you move
the sculpture bottom over it the high points will show clearly. Grind them
down with an angle grinder. Then again grind the bottom against the
concrete/chalk dust. Grind down the high points again. Repeat... You'll
eventually get a flat bottom (and a good sweat).

Bill Weissinger