I line the flap and landing gear bays with cloth, and seal them, too.
Painting bare balsa seldom works well, no matter what you try.
A simple layer of 1/2 ox cloth is easy to apply, and will save hours of time, when finishing off these areas.
A product like Minwax Polycrylic does not smell, and dries quickly. It accepts most paints readily.
I think that you may find that doing this is easy and fast. It's definately worth the added effort.