Use PlastiDip. If you want it REALLY grippy, fling some fine aquarium sand on the wet handle. You won't drop it when it's wet, but I find that for prolonged use it sandpaper's my hand (who'd have thought!).

FWIW I plastidip a LOT of stuff I use in my outdoor/survival kits: CRTK skeletal knives, Zippo lighter, some flashlights, etc. I either dip, or use a disposable 1/2" paint brush and paint the gunk on.