Weathering aluminium
Ferric chloride is a nasty,savage substance. I've used it and it does produce a weathered look, particularly if you allow it to chew up the edges of the metal.
Its seriously corrosive and you need plastic tongs, masks and pots.
I read an article in Narrow Gauge and Short Line Gazette which recommended something called Rottenstone, which I believe is an abrasive powder.
On rust generally, I've seen so many rusty roofs and tanks around the bush that colour is a moot question. I've bought little bottles of builder's powdered chalk and added it to tube acrylic artists paints of the raw umber, burnt sienna, oxide red, oxide yellow, range and obtained nice, grainy rust qualities.
I'm currently wondering whether anybody produces corrugated iron looking material in styrene as I'm frustrated with the problems related to cardboard and aluminium. I can paint styrene, sand it and cut it and it behaves.