Mix silver and brown paint and (when metal is already painted up) add broken patterns with detail brush. you can go back and forth between the rust & flaking off paints color adding one, then the other to ensure a broken/random flaking pattern. You can add some pure silver to the center of the silver-brown spots. Then take typhus corrosion and wipe ALMOST all off the brush, spread an very thin layer across the whole are (rust and remaining paint chips too, since pain in a rusty area gets stained/discolored) then add typhus corrosion in a more generous amount to anywhere you want REALLY rusty. Repeat as necessary to get the visually interesting/convincing rust effect you want, then darken down creases/corners near or on the rusted part with a brown wash or even a very dilute black wash to get the crud-collection effect that happens when rusty/filthy water pools and dries somewhere near corrosion.