Dennis,
 
I don't use dope, but I do use Minnwax lacquer sealer which has similar properties to dope.  It is sandable. I would think you could put Balsarite over it, but I have found that the Minnwax will soften with the heat iron and adhere the covering quite well.  I apply two coats and have used this technique on the past two aircraft I built to apply Polyspan, which has no glue impregnated on the cloth to begin with.  It worked very well.  You also might try white glue.  I have used that method with pretty good results, but I think the lacquer is easier to work with. Hope this helps.

Paul