Well, here's a before and after pic.  If I shrunk 
the pics any, then most of the spots on the scan 
disappear, so please be patient.

Left: Before, Right: After

(Note: I couldn't quite get all the areas due to 
the way certain layers were stuck to each other.)

As you can see, most of the spots are gone.  
Although cloudiness is easiest to see over 
characters, if you look close enough, you can see 
most of the spots are gone from the background 
areas as well.

Since this cel had 5 layers, there were 10 
surfaces to clean.  First, I used mineral oil 
(NOT MINERAL ***SPIRITS***)  to dissolve the 
spots; then I wiped off the excess.  Mineral oil 
leaves a greasy residue, so then I went over the 
surface with soapy water and wiped off the 
excess.  Soap itself can leave a residue, so then 
I went over the surface with just water.  Wiped 
the water off immediately afterwards with a photo 
cloth to prevent warping.  Repeated for each 
additional surface.  (This cel took 3-4 hours to 
do.)  Of course, it's impossible to get certain 
areas or entire layers due to way cel layers get 
stuck.

In a nutshell, 1) mineral oil, 2) soapy water, 
and 3) water.  Any long term effects from the 
cleaning?  I've done this with several cels 
before.  Now looking at the first one I cleaned 
this way, I haven't seen any problems after half 
a year's time.  No warping or anything.

For people who want to attempt this, experiment 
on a cheap one-layer cel first.  I'd try the Zero 
Ryoko BG cel I have next, but the way the layers 
are stuck, I wouldn't be able to do much except 
the very front and the back of the cel setup.  
Oh, as for airbrushed layers, avoid cleaning both 
the front and back of the airbrushed areas just 
to be on the safe side.  Later.