I have experimented with Tamiya Gloss paint thinned with laquer thinner. My plan is to apply this over an etch primer that have been reccomended for use on brass models. So I have to work out a formula for the blue and red that can serve as the base coats.
By the way, Tamiya goes through the airbrush like a charm when thinned with laquer thinner, and it levels out really nice.
As for the state of the engine, the blue is quite faded, and if you look closely you can see rain streaks quite clearly. The blue is also somewhat uneven in tone, so modulation or fading with oils might be the way to go. And I certainly want to err on the dirty side!
As you write, getting a durable finish is part of the challenge. The engine will not receive rough handling, but It will be operated and moved around a bit. And brass is not the easiest metal to get paint to stick to.
The thing I am really wondering about is the wisdom of using a blackening solution as a "primer". Martyn Welch, an extremly talented british modeller advocates this, and he should certainly know what he is doing. Check out mr. Welch´s work, you will not regret it: http://www.martynwelch.com/
Yesterday I blackened my pantographs, and by using a very diluted solution (around 1 part of Birchwood Casey brass blackening to 8 parts of water) and brushing on several coats I avoided the black powdery stuff that oftens form on the surface of the metal. This was a really slow technique, but it seems to have paid off.
So I have to do some tests with Tamyia over primer vs. Tamia over blackened metal. Hopefully, I will also get my blaster setup going soon, I feel that blastering some of the brass components might be the way to go.
I´ll keep you posted!