Galoots:
  With all the knowledge on the porch, I'm surprised someone hasn't
already mentioned this - or perhaps I'm just early.  In his book
_Understanding Wood Finishing_, Bob Flexner has a table that gives
solvents and thinners for various thinners.  In my paperback copy, it's
on page 142.
  He distinguishes between solvent - will dissolve a cured finish and
thinner - thins a liquid solution.  He does go on to say that the same
substance can often be both a solvent and a thinner for a finish.

Substance					Solvent for			Thinner for
mineral spirits, naphtha,		Wax					wax, oil, varnish
turpentine

alcohol					shellac, lacquer,			shellac
						water base

lacquer thinner				shellac, lacquer			lacquer catalyzed lacquer
						water base

glycol ether				shellac, lacquer, 			water base
						water base

water						- - -					water base

The accompanying text goes into quite a bit of detail, explaining what
the table only touches on.  He explains how to methodically test an
unknown finish to determine what it is.  It's a good book, worth
whatever it was I paid.
Charlie Rodgers
Clinton, Maryland