John did a test a while ago on how to strip off paint on old models, both metal and plastic. A link to that article is here. His conclusion was that using Simple Green and letting the models sit, submerged, for 48 hours would get good results.
This week I received a purchase from Ebay: the old set of Commissar Gaunt and his cronies, a really cool set from the nineties. Anyway they came partially painted and completely primered; here they are:
Using the Simple Green did not prove to work with these models. I am not sure if it had to do with the kind of primer that was used or what, but I had it sit for two days and still had hardly any luck. So I went to Kragen's and got a can of carburetor cleaner for 3 bucks.
It is highly nasty and toxic and you want to use it with all kinds of precautions: rubber gloves, well ventilated OUTSIDE area (this time really do it) and maybe a little face mask. It is NOT RECOMMENDED for kids, ADULTS ONLY on this one, I should say. Anyway, getting back to the story. All that effort paid off handsomely for me: I sprayed into an old glass cup until it was half full and then had these models sit in their bath of carb cleaner for 30 minutes. I soaked them in water and brushed off all the paint without the slightest effort.
(Of course, I would not use this on plastic figures)
So when in need, you have to sometimes go for the big gunz and get the job done right (and quick).