What method is best for filling voids in balsa. I have used balsa filler and found it to not work well on any void that is deep. I also mix sawdust with clear airplane dope but does not work well. Any options would be helpfull.

Balsa filler wont work well if there is nothing in the void to back it up.If I come across a puncture of some sort that needs to be filled,try glueing a piece of balsa behind it then add filler a layer at a time.I've also used micro ballons with epoxy to fill in deep holes,then went over the top with filler so you dont have to sand the epoxy.

One thing that works well with dings on balsa is to place a drop of water on the ding, and apply heat with a covering iron. The ding will completely disapear.

Fillers only really work well on small dings... When its a larger void, you need to fill it with a scrap piece of balsa adhered with a mix of epoxy and micro-balloons. (you can also use talc or corn starch instead of microballoons...)

For the large void:

Cut the scrap piece to fit as best as possible. Mix 30 min or 2 hour epoxy (5 min won't give the needed working time) Pant the void's surfaces with epoxy and paint some epoxy oon the scrap that is being used for filling. mix in some microballons to the remaining epoxy and form a paste. Squish the past into the void and then squish the scrap in and firmly press it down. Smooth out the microballoon mix that squeezed out. (scrape off gross excess) When that cures, sand smooth.

I second the suggestion that if it's big enough to call for scrap balsa, scrap balsa is the only way to go.

It was also suggested to me to use wall spackle (the light, water-soluble stuff). Apply too much of it moist, wait for it to dry before sanding. So far, it's applied fairly well and sands down smoothly. Of course, I haven't flown the thing yet...

If it's just a ding, the earlier post about dampening the spot and hitting it with the heat iron is right on.
If that won't get it out, use some Lightweight spackle-available at any hardware store. Two bucks for an essentially "lifetime" supply (pint).

Of course, a real hole has to be repaired first, than it can be smoothed and sanded flush using a litttle spackle to fill the imperfections.

Obviously all sanding should be done with a proper sanding block. The spackle sands more easily than balsa and can be feathered to a very thin edge. It works better to sand the spackle in one direction only. If you go back and forth, sometimes the spackle dust will "pill" just like eraser crumbs.

If the spackle dries out, you can add a few drops of water to the container and stir it. When you first put the spackle on the wing, it seems to help to massage it into the spot. Fingertips are good for this-Spackle washes off easily. After the spackle is worked a bit, it liquifies and attaches to the wing surface much better than when it first touches the wing.