Winning colours paint and stain remover (avail from Cambodian Tyre, Home Depot, etc)
$10 for a big squeeze bottle (over 1L). Completely safe on plastics, FG, etc...
Good method is to put some of solvent on rag, wrap rag around area. At this point, you can even just put the part and rag in a bag and let it sit.
After letting it sit, come back to it with a brush, 3M pad, or what have you. Paint will come off plastic handguards easily.