That may well be true of Sig dope. A manufacturer can formulate dope to have almost any shrinkage. Most provide a choice of tautening and non-tautening dope in both nitrate and butyrate. It is not something that is inherently characteristic of the two types. I mostly use “Certified Coatings”, or “Randolph” brands because that’s what my local supplier carries. I have also used “Brodack” and in the past a lot of “Testors”. As I said above, there is more difference between brands than type, but the differences are not significant.

The foam brush might be a good idea, Brush hairs have always been a problem, especially with a new brush. I'll try it next time and see.