Andi,

If you are referring to the ugly , whitish, filmly material you might
see--the times I would get it is from not properly washing slides after NFR
counterstain.  If you go directly into alcohols or wash too fast in water
before alcohols, the aluminum sulfate from the NFR will precipitate.  It is
soluble in water but not in alcohol and ends up as a film.

Ray
Seattle, WA