ORIGINAL: TomCrump
Lacquer thinner should clean your brush.
Balsarite works well for fuel proofing.
You shouldn't need Balsarite under Monokote or Ultracote. Both have good adhesives applied to them. I find Balsarite useful under the ''tex'' coverings, where it provides extra insurance.
I'm with Tom on this one. I only use Balsarite when using fabric type coverings like 21st Century and Solartex. And acetone should easily clean your brush. In fact I thin Balsarite a bit with acetone before application to an airframe.