I don't usually use balsa wood to build furniture. Basswood is my favorite choice.
Today I read that you can seal balsa wood with a mixture of 2/3 white glue and 1/3 water. This keeps it from warping and is supposed to help it take paint better.
I'm working on the frames for my little sofa and ottoman. I sealed the wood using this technique. It dried nice and flat with no warping at all! I didn't even have to weigh it down.
I plan to paint the frames black and then add Martha's metallic glaze. Hopefully I will have the finished furniture to show you later?
Hope you have a nice day!