cork block mental block

[Greg Graham]

Simple questions, from a simple mind:  

I have just received a new cork block for rubbing out
lacquer.  One face of the block has square edges, the
other face has rounded edges.  Which side goes against
the sandpaper?  

Does a new cork block need to be flattened before use?
 If so, what's the best way?

I've always just used wood blocks (or a machine) in
the past, but I'm trying to refine the art.  Tips?

Greg Graham, RPT
Brodheadsville, PA