If the spot is bare wood a heat gun will make the fuel and oil run to the surface where you can wipe it off. Just don't burn the wood. Of course, this won't work if it's really close to covering.

To fuelproof bare wood mix some epoxy with denatured alcohol. It should be of similar consistancy to paint. Brush it on and let it dry. I've had a tank rupture and could still fly after sopping up the fuel and replacing the tank because I fuelproofed the tank area.