being a carpenter and painter my latest favorite filler has been Minwax wood filler it's pretty much the same as the type ken recomended just a different brand, I like the minwax brand because it sands nice and smooth, although it is quite hard when dry.
another ding removal tip for balsa sheeting a 50/50 mix of denatured alcohol and water, will puff up the dent with the water and the alcohol evaporates fast so the wood doesn't warp I have found it works great for small dents in wing and fuselage sheeting as well as for softening the wood for bending around curves.