RE: What brushes for dope?
The last time I doped a plane, about 40 years ago, I used Camel hair brushes. It took a while to get all of the stray strands out. I don't know if you can still get them or not.
A friend of mine that dopes all his planes even now and buys dope by the 5 gallon cans, is using the throw away foam brushes. They were not around when I doing dope (Paint dope that is).