I also like to fuel/oil proof some areas that will get hit by the exhaust. Prior to covering & assembly, I oil proof the right aileron where the torque rod is imbedded into the balsa. I also proof the area where the torque rod exits the trailing edge of the wing. I like to use Robart hinge points, so I proofed around those holes on all the flight surfaces. I also proof the fuselage around the local area where the leading edges of the vertical fin and horizontal come together. On my 4*40, these are all areas that are covered with oil after a day's flying. In a perfect world, covering and final assembly would seal these places against oil, but I've learned the hard way that it's hard to be perfect - oil residue only needs the tiniest of openings before is starts to soak your balsa.

Good luck, Gary