On World War 2 military aircraft is was quite common for the paint between the fuselage and the wing root to become damaged by the pilot and ground crew while walking up and down to the cockpit. Although there are several painting techniques used to replicate paint chipping on an airplane model kit, a simple and effective alternative would be to use a silver pencil. These can be purchased at any art supplies store.
Stipple the wing root of the plastic scale model with the silver pencil until you have the desired amount of dots. It is important that you do not draw, or the effect will be lost. Applying dots and tiny dashes in this method will give you a great deal of control over the final look of the airplane model kit.