I have a solution that works great for stipping paint off of metal and plastic. I use this method all the time with my pewter and plastic 40k models. A word of caution, do not use acetone on plastic! It will melt it!! My method is tried and true, and will not harm your model. It is safe to use indoors & you don't have to worry about getting it on your skin since it's non toxic. Also it's cheap, $8 or so at almost any grocery store.
Supply list:
Simple Green cleaning solution
Old toothbrush
Water
Plastic container

1) Put whatever you want to strip paint off of into your container.
2) Pour in enough Simple Green to cover the pieces completly. If they float put something over them to wiegh it down.
3) After a day or two take the pieces, figure or whatever out of the Simple Green.
4) Using the old toothbrush scrub the piece making sure to get in between the details.
5) If the is still paint on the model, repeat the process. Depending on the type of paint used this method will remove most if not all of the paint.
6) Rinse the parts with water while brushing to remove residue and allow to dry on a paper towel.
7) Prime, then paint.
That's all! Have fun and let me know how it went!