Cleaning unclear dusty cels?

I got a muti-cel setup with background, and each cel seems to have a lot of dust/pits/maybe scratches (previous owner did not store it very well...)...any way of cleaning/fixing that? Normally I'd let it be, but it makes it hard to see the background. Paint and lines are great though.

Thanks in advance!

Multi-layer cels have a tendency to rub against one another when stored together without protective separator layers.
Odds are what you're seeing are scratches/abrasions. No amount of cleaning will fix this, unfortunately.
Consider storing each layer separately, or placing a thin layer of plastic between the cel layers in order to prevent further scratching.

Thanks for the info...kinda makes me sad though :(

I don't think I can safely separate the layers...it's pretty stuck in some parts, and some bits of paint are already on the wrong cel. It did have one completely blank cel that I removed (held on by Anime Cel Yellow Tape®), but it was itself very beat up, and it looked better without it.

Any specific way it should be stored to limit that scratching?

Anything that can be safely removed can be bagged separately. Stuck layers can be bagged together... they won't rub against each other now that they're stuck.
If anything is stuck to the background, leave it. The background will likely tear if you try to unstick it.
Once bagged, you can store the cel in an Itoya.
Hope that helps. :)

Mild soap and water will get some of the dust and "haze" off your cel. But, as Gelato says, some of it is damage to the plastic, which is chemically active and develops "pits" even without being rubbed together. If you can separate the layers without damage, yes, do so, but it would be a good idea if you can't to cut a plastic sheet and put it into the empty space that you can pull apart. That will help fumes vent and slow down the rate of future damage.

If you scan the set-up as is, you can often make the background more visible in the scan by amping up the contrast.