Yeah, "Tube60" is right. You need to apply a "sanding sealer" to Balsa before priming. Most primers are going to just soak right into th egrain of Balsa, and you'll probably end up putting several coats on, with less-than-satisfactory results. You can get water-based sealers at any good hardware store, so cleanup is simple, and they aren't too stinky. It's thick stuff though, sort of like a watery Elmers glue, and you need to put it on with a minimal amount of fuss, because it doesnt like to be "gone back over"--it tends to show brush marks if you work it too much.
In the model airplane world, they make stuff called "dope" that is a REALLY thick laquer-based paint for painting balsa, cloth and paper. It's so thick that it doesnt really absorb into the porous materials as much as sit on top of them and then dry into a hard, smooth finish. IT's REALLY stinky though, and a HUGE pain to deal with because it doesn't brush well, and it tends to skin up quickly, making it tough to go back over spots you might have missed. I don't even know if the "good stuff" is available anymore. If I remember correctly, back in the day it was made with toluene as the solvent, which is REALLY evil stiff--volatile, carcinogenic, and flammable as hell. But man, did it make for a great finish on porous materials...