I use compressed air to blow off the balsa dust just prior to covering, then I wipe it down with a tack rag. You can also vacuum off the balsa dust. It is important to get all the dust off before covering and cover it in a dust free environment. Balsarite is not necessary with the plastic type coverings i.e. Moneycoat/Ultra coat but if it'll make you feel better use it around the front end, around the landing gear attachment points etc any where there will be a lot of oil from the exhaust.
What Balsarite does do real well is make it hard to remove the covering to do repairs.