From Testor's web site"
Acrylic Paint Removal
To remove dried acrylic film, you may use PollyScale Easy Lift Off Remover No. F542143. Many modelers have found that this is an effective way of removing the acrylic paint without harming the polystyrene. Apply the remover and leave it on the effected area until the paint starts to wrinkle. Do not immerse. Using a soft sponge or brush, rub the film under running warm water. The film will rinse right off. A repeat application of the ELO may be necessary depending on thickness of the acrylic applications. However, if the acrylic surface has been sanded it will be much more dificult to remove the paint. Please follow the precautions listed on the ELO product label.
Once completed, clean the area with a mild dish washing detergent and allow to air dry before you touch up the area with paint. You can also wet sand areas that do not involve clear parts. Simply apply moisture to the paper and gently rub. Rinse out grit to keep the paper workable.
Enamel Paint Removal
To remove dried enamel film you may use PollyScale Easy Lift Off Remover No. F542143. Please see instructions above under the Acrylic Removal section.
You may also use Easy-Off Oven Cleaner. We recommened the Fragrance Free version of the product. The procedure for removel is the same as the above one for PollyScale Easy Lift Off Remover or ELO.
I just use turpentine, I have a lot of it and it works. If your paint was acrylic you can use alcohol.