The best results I've got so far are by doing it in the following order (after building the entire structure and filing/filling/sanding to get everything looking nice):
Spray bomb with primer -- I use Tamiya light grey
Roughly mask off the roof and concrete top section (just to save overpainting work) and spray bomb the woodwork colour (probably two coats with some fiddling in between).
Brush paint the lintels / cornice elements whatever colour you prefer (if you want them different from the woodwork colour. You only have to be careful not to paint onto the woodwork, it doesn't matter if you paint on to the brickwork.
Then paint the roof the initial off-black and whatever other elements need doing.
Then, lastly, paint the brickwork. I brush paint this with whatever colour (or slight colour variation) the bricks are supposed to be. Because you're painting a (comparatively) large area up to an inside corner (cornice/lintels/windows), this is far easier to get an clean break on to the lintels than doing it the other way around.
Then I wipe over with a light grey fine grout (household grout as used for tiling) for the mortar, and rub off the excess when dry. The grout, being porous, takes inkahol more than the painted areas. And it's easy to poke into holes and rub off where there's too much.
Light inkahol over the lot, giving the base coat of weathering, and dullcote the whole thing.
Then you can paint the inside walls, insert interior decor, glaze the windows and do whatever else you want...
Then signage and detailing, downspouts, roof details or whatever
Then final weathering to taste...
Charles
Edit: I didn't answer your questions, did I? No I don't use an airbrush, I only use enamel spray cans and enamel paints by brush. Most weathering except for inkahol I use weathering powders.