RE: Plasticized dope mixture
It sounds like you are "mixing" processes. You would add castor oil to the thinned dope to plasticize it and help keep it from hardening too much and cracking.
You would add talcum powder to the thinned dope to make your own sanding sealer.
I don't recall doiing both to the same coat of dope. Plasticized dope would be used on silkspan. Sanding sealer would be used on uncovered wood.
But it has been probably 55 years since I did either, and i could be wrong.
Maybe contact Brodak, they seem to cater most to silk (span) and dope type finishes.
Good luck.