With old kits it is sometimes a case of work with what you have but for wing sheeting you might want to consider the harness of the wood. I have had some balsa that looked more like mahogany it was so hard. It didn't want to bend even after being soaked in water/ammonia. Don't go so soft that you put your finger through it picking up the plane (been there) but softer wood will curve around the leading edge more easily.