Balsarite WILL help iron-on covering to stick. It works. It is not really necessary in most cases, though. You need to put a LOT of it on an area like a firewall in order for it to be fuelproof. One coat is not.
It has absolutely nothing to do with covering with silk or silkspan, and should be nowhere near. Neither should polyurethane.
Do it right. Go out and get some nitrate dope. Sig sells it. Use the nitrate to attach and fill the silk, then finish with butyrate. Nitrate is much stickier than butyrate, and fills much better. It's not fuelproof, though. I think the reason a lot of people find silk and dope difficult is they do not know about nitrate.
And check out the Brodak butyrates. They work categorically better than the Aerogloss stuff. Do NOT attempt to use aerogloss white or yellow. You will be sorry.

So.....no, balsarite is not nitrate dope. Don't try to use a substitute, not laquer or anything else. It might end up being totally incompatable with whatever you use next to prime and paint. Don't reinvent the wheel, here. You could end up totally ruining your model. Cover it with nitrate dope. Prime it with Pactra Prep or Brodak dope based primer.
Check out my gallery page, and see that I do a lot of silk and dope and know what I am talking about...good luck!