I've got a friend who's used Ace brand VM&P Naptha a few times now with no complications.  When he did the evap test upon acquiring the solvent there was no visible residue left over.    I wonder if the fan blew some crap into the mix to cause the residue,  my buddy let the glass plate air-dry when he did his test.  The volume of naptha tested was ~25ML so it evap'd pretty quick.