Sounds like polish will work to remove the oxide outer layer. You may also consider the special polishes intended for autobody plastic, but the safe move would be a simple abrasive type like the Maguires.

If that isn't good enough, I like the Krylon for plastics spray paint. Goes on smooth and sticks well. I don't imagine it hurts the underlying plastic, and becomes a UV and oxygen barrier. Never used it for delrin, though.



Generally speaking, there is a lot of old plastic in the world on very collectible old consumer goods, firearms and toys. Despite a lot of motivation, no one has a magic bullet to keep that stuff new or put life back into it. It just wasn't made to last forever.