For simplicity's sake, I'll refer to Elmer's Carpenter's Wood Filler as "CWF".
Years back, I used sanding sealer to fill balsa wood because that's what we were told to do in the kit instructions. Many old school builders still use it and swear by it.
I now use (and swear by) Elmer's CWF. If you read the forums, some still call it by the old name of Fill N' Finish or FNF. Elmer's no longer makes it under the Fill N' Finish name. Essentially it still does the same thing - it easily fills the balsa pores.
EDIT: Buy the CWF with the orange lid. Stay away from the stuff labeled "MAX". It is impossible to sand!
I use the CWF for good reasons:
1. CWF doesn't smell as bad as sanding sealer.
2. I can apply CWF indoors.
3. It's a lot cheaper than sanding sealer.
4. CWF won't load up your sandpaper. The sandpaper lasts many times longer.
Some complain about how much dust is left after sanding with CWF. I sand a lot of bare balsa, there is dust everywhere anyway.
In the past I've just scooped out some CWF into a small Tupperware style container and added water. I'd go back and forth adding a little CWF or water until I thought I had the right mix.
For this blog, I thought I'd try to find the correct ratio of CWF and water for a good, brush-able mix.
The mix of CWF and water was not what I expected.
I used measuring spoons a baker would use to add dry ingredients in a mix.
I started with one level tablespoon of CWF.
I added one level tablespoon of water. It was stirred to mix. The lid was snapped on and shook to mix a little more.
It was very thin. If brushed on thin balsa, if would warp the wood.
Another level tablespoon of CWF was added. After mixing, still a little thin. More CWF was still needed.
In the end, here's a workable mix ratio:
1 Part Water
2 1/2 Parts CWF
I can hear the reader now: "That's not the way I do it!"
Of course it's not - everybody has different technique that works for them. But, this could give a first timer a good place to start.