Some g3 stains can be removed with acetone or magic eraser, since their plastic is much softer. It is, however, also easier to damage, so be careful! Some G3's also seem to have the plastic sort of layered, (mostly white ponies) and you can 'peel' the top layer of plastic if you scrub too hard with chemicals or the magic eraser. It looks like a blister afterwards, it's very noticeable, so don't rub too much in the same spot, or heat them up prior to scrubbing.
Here's what I do with my troublesome G3's:
Firstly they get a good cold bath with antibacterial soap and a toothbrush. Then I sort out any hair issues with a quick condition and straightening/curling if necessary. I always use conditioner, whether they need it or not, because I'd rather comb it out and prevent any further damage to the hair, than risk stretching it dry and causing frizz.
Then I get out the magic eraser. most ponies benefit greatly from a quick wipe over with it, it picks off most residual dirt, and takes off a fine layer of plastic making ponies brighter. It can also remove some stains. If the magic eraser doesn't work, or there's actual paint/polish/etc on the pony, then out comes the acetone. Now I use a q-tip for acetone, it's pretty precise and doesn't hold much liquid. Make sure you get the excess off so it doesn't run all down your pony - acetone WILL take off symbols/blush/eyes! Please test under a hoof first.
Hopefully that helps. Best of luck!
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