RE: Balsa wood filler?
Shane,
Go to the lumber yard, Menard's, Lowes, etc. and look for "Red Devil" brand "One Time" light weight spackle. It is about 1/10 the wt. of ordinary spackling compound and extremely easy to sand.
If you are using covering, (MonoCote/UltraCote), you may want to put on a little BalsaRite. If you can, avoid putting your iron Directly on the filled area.
I've used this stuff on everything from trainers up to and including our 40%ers.
Good Luck,
Steve