Ron,
I've found that the best method is to seal the bare balsa with good grade of finishing epoxy such as Pacer Z-Poxy.  I usually mix enough to do 1 panel and brush it on as thinly as possible.  Apply a little heat to thin it and then use as many paper towel s as it take to wipe all the excess off.  You don't want to leave any excess on the panel as it just adds weight.  Let it cure over night and proceed with your normal finishing procedure.
Alan Resinger

Ron,
I usually use one of the cheap throw away white bristle brushes that you buy at home improvement stores.  I trim the bristles square to make it a bit stiffer.  They are usually about 3/4 inch wide.  After the epoxy cures I lightly sand with 320 grit and then apply a couple coats of butyrate dope before applying CF veil.  Then it is more clear to make the surface hard before going to a couple of coats of filler.
Alan

Thanks Randy,  seems the old silkspan still has its merits, by the way what type of dope do you use for the application butyrate or nitrate ??