I have relatives that have a craft business . They make lots of old furniture and crafty things from old doors ,windows and some things from metal. Lots of the furniture is brushed with a dark flat black paint . Once it's dried they simply sand and buff the edges with a light sand paper and it gives a nice weather worn look. In your case I would try some trunk finish paint that gives it a ripple and rough sand paint look , perhaps in a dark brown , chip some of it maybe in places ,sand it here and there in spots and add the vinegar and salt spray and see what you can create. But I know what you are referring to , smooth new mild steel cold roll steel rods with surface rust stick out like a sore thumb and just don't look right when trying to match the rest of something with years of exposure and weather.