You could get some thin super glue and rub it in wearing rubber gloves. Something like bone or stag will soak up a bunch, not so much the top surfaces but probably on the back side, avoid built up in the natural part. Once you have it so it stays on the surface it will sand and shine up nice.
I have used it quite a bit on some natural woods, Oosic, bone and antler knife handles.
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I thought this was going to be a "My dog ate my gun" story.
Nice grips.