If your gonna use Tamiya smoke for weathering, just use it out the bottle & dry brush the landing gear otherwise airbrush it the same way as any paint, thin then build up the colour, I think your refering to a sludge mix which is different weathering technique altogether,used mainly for panel lines, I'm sure I'll be corrected if I'm wrong.
Scott.
www.freewebs.com/scottellis - My hanger.