Chuck,
I have found Balsarite is not needed to cover an airplane and it actually makes it harder to get a good smooth result. The one exception is when doing repairs and the wood is oily. Another area where it is good to use even on new construction is around the nose of the aircraft or an area where the balsa is curved, or a compound curve, etc. The fact that you have old Monocote is a plus. I have used 10-15 year old Monocote and it works as good as new, etc.