RE: dope question
The adding oil was generally used for turning dope into a micro-film ocvering for super-light indoor models. There were all kinds of interresting formulae on how to alter dope for this purpose. All included some oil, I know of one that involved 10% beer, 89% dope, and the other 1% was oil and other stuff...
Butyrate Dope hase less than half the shrinkage potential of Nitrate Dope. So generally for painting the wood ofr fabric covering, you don't add a plasticiser to buterate dope.
If Dope is being applied over some other paint... yes you can have severe crazing of the finish as the dope ages. You won't fully prevent that no matter what you do. The laquer base of Dope just does not fully bond to other paints. (not even to automotive type laquers.)