ORIGINAL: bps
I doubt you will be able to get rid of the grain of the balsa. On top of that it will take a bunch of filler, sanding and so on. You will also run the risk of cracking when the balsa absorbs moisture and then dries out. Koveral and poly seem like the best option to me with the least amount of work. It will take a lot less filler to fill the covering weave than it would take to fill the basa grain in my opinion.
actually you can get rid of the pores and grain in the balsa. after i put a proper coat of sanding sealer i then paint a few coats of watered down hobbylite balsa filler all over the project. then i sand it all at once. no filling just painting. and sanding.
and something i've yet to try is some of the scale builders use lightweight drywall mud to skim their project. since these guys use epoxy on their cloth it supposedly seals the mud against cracking. i haven't tried it since i use water based polycrylic on my cloth.