I've been doing the same thing. First thing I tried was Dot 3 brake fluid. It works on models. Models are made of styrene, and it doesn't hurt them. Depending on how long you soak or how hard you scrub, you probably will damage the factory paint. I've never had it hurt the plastic on the styrene based ARAH Joes. I've had mishaps with TMNT and Wildcats figures. The TMNT paint dissolved, and whatever plastic they made Wildcats out of crumbled to dust.
After these mishaps, and dry skin, I tried Simple Green. I used the concentrate from a gallon bottle, and poured some into a peanut butter jar halfway full. I would highly reccomend the Simple Green! It works better than btrake fluid, and doesn't irritate your skin. all manner of paint (spray, enamel, acrylic) comes right off after soaking for a day or maybe more for stubborn paint, scrubbing with a toothbrush, you may need a toothpick or sharpened chopstick for crevices.
ARAH parts. Like new, no damage to factory paint.
Modern joes. Like it never happened.
Robotech figures.
Factory paint scrubs off with the rest of it.