Let's face it, I'm an experimenter by nature. I think the real key to using /anything/ on a doll is to have a good understanding of what it's going to do and how... what can go right and what can go wrong. I see a lot of things I sometimes use on dolls get a lot of "OMG DON'T" from some, but I think the key is caution and knowing how much risk you're willing to take. The two biggest ones?
Acetone (which I have used, but only once as it's honestly not that useful). Acetone affects the plasticizers in vinyl. While it might be useable to remove some doll makeup providing you use it quickly and then be sure to rinse VERY thoroughly, I really don't find it worth the bother. I've never damaged a head with it, but since it usually just smudges modern factory faceups around (meaning a lot more time before you can rinse, which affects the safety) I don't have much use for it either. (I remove factory faceups with a dremel and a light touch... rubs em right off! )
Acetone, though, does have one interesting trick due to the plasticizer leeching that I may want to try on a spare junk barbie knockoff head someday: it also makes the vinyl shrink. If you completely submerge a vinyl head in acetone, it will first swell up, and then shrink down. I learned of this because of people using it to decrease the head size on the badly proportioned 12" Star Wars figures. I've also seen partial resizing done this way by just soaking the bottom half of the head in order to shrink a neck hole slightly (with the side effect of a bit of a pointier chin). However, it's very very likely to remove all traces of faceup. Also, due to the loss of plasticizer, the head is likely to be a bit harder and a bit more brittle, proportionate to the degree it was soaked. Some heads reportedly can split during the shrinking process, and I have a feeling you wouldn't want to try to reroot a shrunken head... But someday I might try this on a junk knock-off Barbie just to study it firsthand.
However, one thing I'm more curious about and might try as far as acetone is, because it removes plasticizers, I want to see what it does for old plastics that have started to ooze their plasticizers. You might know what I mean. Old stuff that becomes sticky and there's really no known treatment for it. While, thankfully, I have no dolls like that, I do have an old pair of Cabbage Patch Kid shoes that are horribly tacky. I can't put them on the dolls like that, and can't really do anything with them other than leave them in a baggie or toss them... so they seem perfect experiment fodder. Worst case, they get brittle and break and have to be tossed anyway. Medium case they still get sticky afterwards, only they now can't be put on a baby Cabbie due to sticky instead of not being put on an older Kid. Best case, they wind up baby Cabbie shoes and no longer ooze... at least for a good while. As I said above, with the 'don't' stuff, it's all about the risk you're willing to take - and if something's looking like a candidate for the garbage bin anyway, there's not really anything to lose (plus, I just hate to throw things away until I've tried EVERYTHING. XD )
Another thing I've recently seen arguments against is 10% benzoyl peroxide acne creams. Unlike the acetone, this one I DO keep on hand and use. Without it, I'd have likely never removed all the gelpen from Ivy's face, not to mention treated her Cabbage Pox.
The key with it, is to remember that it is a bleach, and that it is just as possible to bleach the vinyl as it is to bleach the stain you're trying to remove. The key to success is which one's going to be more susceptible, and that can vary wildly between different manufacturers and even different batches of vinyl. Any time I use it, I first make sure to test an inconspicuous spot (like on the edge inside a head) for 24 hours to make sure the vinyl batch isn't one that will have some weird extreme reaction. Then, after applying to the stain, I wipe if off at least every 24 hours and check. If I see even the slightest hint of unwanted changes to the vinyl, I stop applying it. Usually, what I see is the stain being just a hair lighter... Getting the stains off Ivy took a bit over a week of daily applications this way, and I halted with a couple of her Cabbage Pox still faintly visible (if you know where to look) because I noticed a very faint lightening in the vinyl around the pox spots.
A bad reaction to acne cream or really any other bleach is likely to yellow a doll or even turn them orange. Apparently, the pink tones in the dyes used in vinyls are more sensitive to bleach than the yellows/oranges. This is also why some vinyl dolls will turn yellow in the sun - they're bleached by it and the pink tones went first.
So, are there any things you use on your dolls with reasonable success that tend to make others flail when they hear you do it? What tricks or precautions do you use to avoid the disasters?