Well, the discussion of Biox piqued my interest, so I went looking for
some information about it.  I found a couple places that mention it.
One said that it has a pH of 2.2 (rather acidic).  The other said that
it was a solution of citric acid and water.  Seems reasonable and smells
nice too.  I didn't find anything about it being an organic compound
except that it is related to lemon juice.  So, if you want to dunk your
disks in acid, why not just put a couple drops of lemon juice on the
white spot and see what happens.  I think it would be wishful thinking
to believe that putting acid on a zinc disk will not harm it.  I'd keep
some baking soda handy if you try it.  For really rusty disks, use lime
juice (Key lime juice, if you're in Florida).

Regards,
Craig Smith

P.S.  Stella disks should clean up pretty well with the Evapo-rust I
suggested yesterday.  Notice that this company foresaw that cleaning
would be a problem as the disks got older, so they got rid of those pesky
little projections on the back, so you could clean them more easily.
Take that, Regina and Polyphon!!!