Depends what paint it is really, with faskolor airbrush paint you can usually soak it in water - but if it's fully cured it won't come off that easily. Bit of wire wool can sort it out I guess.
Other things like brake fluid can take the paint off lexan shells - I've never done it on polycarb but I've had a shall that's been done this way. It can (will) cloud the lexan - so if you are wanting clear windows you are out of luck. any bits of paint left can be sanded / wire wooled, and unless you're using particular types of paint like chrome, it shouldn't show when painted.
You can also use ovencleaner / caustic soda - but I'm not sure how this effects lexan. I think it can cause it to become brittle but if you're careful it might be OK (don't leave it soaked in the stuff in other words)
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