Paint stripper works (duh!). Acetone works. Brake fluid works, but I'd use it sparingly (some rubber o-rings may not like it). And Castrol GTX degreaser - as well as similar caustic "biodegradable" cleaning products - will remove most enamel paints (this is particularly helpful on plastics, where something like acetone would not be safe).
The best way to use any of these is by applying a small amount to a rag, and then wiping off the paint. Have a lot of rags (or thick paper towels) handy. Wear gloves, and do this in a well-ventilated area. Keep the gooey paint rags away from the lenses; in fact, a bit of painter's tape over both lenses is a good idea.
