RE: Plasticized dope mixture
I would not bother with plasticiser unless I was doping an open frame structure. the balsa will be solid enough not to flex enough to cause the dope to crack (in my opinion).
The silkspan will fill most of the grain and you will not need too much dope. The talc and dope does fill the grain and weave well. Be careful sanding the edges not to cut through the silkspan. I mainly use pure silk and although I don't get a super shiny finish, I use coloured silk and finish up with a nice "sheen" that looks good on an old timer.
Regards