RE: How to take ding out of wing tip?
Can't see that photo very well. If it's just a dent, try this before cutting:
Wet the dented area and let soak in a couple of minutes while your covering iron heats up.
Apply the hot iron to the damp balsa. It'll sizzle a bit and will take its original shape as it dries.
In this case, don't bake it too hard because of the foam.
I'm amazed at how well this repairs dents.