I share because I care!
Seriously, weathering metal is fun, even if it's kind of a tedious process, with a lot of trial and error. The end result is usually really impressive, and is automatically more realistic than any "fake" weathering done with paints or washes or such (unless done by a REALLY talented artist) I used a TON of different methods, most of them not really effective, but along the way I stumbled upon techniques which I'll use without fail in the future. The key is to find what will chemically react effectively with the metal being weathered; with a prop like this it's vital to make the parts look like they've been exposed to the elements for decades, which can't be convincingly done by simply damaging the part with steel wool or something. It has to look natural!
The most efficient methods I've found are:
For steel, a bath of water saturated with salt, with periods exposed to air to let the part dry and the rust to really settle in. To darken steel, there's Rt-S25 blueing solution, which will give the steel part a nice, even, blued finish, which may look too perfect for some. The aforementioned "oven on broil" will darken the steel in a more random pattern.
For brass, Brass Darkening Solution will do miracles to, well, darken the brass.
Aluminum Black will also darken brass slightly and randomly, and more importantly a lot less evenly than Brass Darkening Solution. Just apply with a brush or a Q-Tip.
For copper, Brass Darkening Solution will also darken the metal evenly. For a more random finish, use liver of sulfur, or if you're into gardening, lime sulfur will do just as well.
Just apply with a brush and rinse.
For aluminum, Easy Off oven cleaner will darken and dull an aluminum surface, fast. It'll also do nicely in lieu of paint stripper in a pinch, and supposedly removes anodizing, but I never tried this. Obviously, Aluminum Black will blacken aluminum really fast, and it's perfect to repair scratches and such.
For all metals and machined parts, an ammo tumbler will deburr the brand-new parts and give the piece a finished and slightly worn look that can't be replicated otherwise. I don't know what I would do without mine. It can also be used to strip or clean a metal part that has be weathered too much for some reason, or to wear out a part that is too gloss, etc... The possibilities are endless.
Of course there's always the good old methods, i.e. damage the parts by rolling them around in gravel, or denting the edges by dragging a hammer head across it, etc... Some metals are made softer with heat, which is how I made that nice dent in the wind vane seen above. By accident, might I add.