> The trick here is to polish the board as usual, but then instead of
> using sandpaper to roughen the surface, dip the blank in etchant
> instead.  After some small percentage of the total etching time (by
> trial and error), remove the board and rinse thoroughly under running
> water.  Dry the board with warm air (even a lint free cloth might snag
> some debris in the rough surface), then transfer the toner to the
> board with heat as normal (the surface will probably look terrible,
> but that's the idea).  After etching, remove the toner, with solvent
> preferably, then use steel wool or rubbing compound to polish the
> remaining copper.

This is what some friends of mine used to do; I'd always thought it was
simply to remove oxidation or contaminants, though if it pits the copper
that might be helpful too.