I'm pretty sure using paint thinner will strip ALL the paint off your gun, and potentially ruin something.
That's what happens when you buy a painted gun, its never going to go back to the way it was...
You can buy new external parts or a new gun.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN