Our family has been using one commercial stove black, that contains whole lot of graphite, has to mixed well, rubbed, heated to set (or it will stick anything that touches it) and when it cools, brushed loose stuff out. It produces traditional fireplace black color.
They sound pretty big objects and you may not want to heat them up, they might contain old paints and stuff, very probably lead, not a good idea to heat.
I pretty much just would take away the biggest muck and put them into a display on mesh they had....to show some functionality. I would leave them "dirty", they were used that way.
Mild phosphorous accid (easy to find mixture of citric and phosphorous accid mixture is tea ketlle lime rover from grocey store) will produce steel grey you are after, just need to very clean surface first.
Pekka