I don't mean to step on Ty's toes here, but DO NOT use spackle for filling dings in balsa. Ive seen too many times, dope delaminated from poor adhesion. The finish will look fine for a few months, then after sitting out in the sun, bubbles appeared in the finish.
I use automotive spot putty with is a very thick laquer primer. Very easy to sand and since its a laquer base, dope sticks very well
As Ty suggested, you can steam out small dings with water. Here is a cool trick my dad taught me.
Use a MonoKote iron to steam out dings. Put a couple of drops of water on the imperfection and place the iron on the ding, This will cause the wood to swell and the ding disappears.
Larry, Buttafucco Stunt Team