RE: Aero Gloss Sanding Sealer...
Without some type of covering, the balsa WILL "work" and eventually crack the paint.
IF you insist on using dope, the most stable filler would be to mix regular talcum powder with clear dope (1/3 talc, 2/3 dope ) mix well and apply a coat. Let dry (anywhere from 2 days to 2 weeks to completely dry ) , sand, and repeat until the grain if completely filled and smooth (typically 3 - 6 coats of filler ). Then apply your colored dope letting it completely dry between coats (typically 2++++ coats depending on the color ).
As DR1 said, glass it. Use some .5 (1/2 ) oz glass cloth and adhere it with water base polyurethane. Fill the weave with a mix of micro-balloons and wb poly, sand, then prime, sand, prime again and paint. Easy to do, about 1/2 the weight of using resin and about 1/2 - 2/3 the weight of using a balsa filler, not to mention the wb poly dries in a couple of hours to overnight at the longest.