Hey all,
Getting quite tired with the usual less-than-satisfactory job (in my opinion) that Elmer's Fill-N-Finish gives; I borrowed some material and techniques from my "other side" (plastic scale modeling).
I used Bondo automotive Glazing & "Spot Putty", the red stuff, which is the same product as Testor's red "filler putty" (both being made by "The RPM Company"). Pre-sand the balsa as usual, then paint on a heavy coat of the Bondo/Testor's putty, that has been thinned heavily with Testor's Liquid Cement. Thin it to the "consistency of milk" (to use an old cliche); and brush on a heavy coat. Although it has a far less chance of warping the balsa, as opposed to something water-based, go ahead and coat both sides at the same time. Thinned with the liquid cement, it dries very quickly. Within the hour, it is sandable. With just one brief attack with some 400 grit...
...smooth as *glass*!!