Sorry, I have completely forgottent to present here my technique for weathering...
So, after the primer I airbrush metal color (generally alu) as base. Then with a sponge, I make some "blotches" with maskol (or equivalent products, like Neo from Gunze) at the locations the paint of the aircraft is suffering the most from manutention, vibrations, etc.
I airbrush the final color and, after having let the time needed to dry, I gently remove the Maskol with a Q-tip...