The thing is SEM doesn't recommend hand painting. I assume it has to do with flow and brush marks. But as this is all hidden, faint brush marks would be acceptable. So brushing in the flat black would be my first choice.
Alot of unknowns. And they dont have a flat black in a can. A flattening agent has to be added. If I knew some other paint would work it would make life easier. only want to do this once.