Reference! Reference! Reference!
Cannot state how important this is. Wasn't it Caravaggio who said "If you want a thing to look like a thing look at the thing." Google images immediately gave me this when I typed in "rusty painted metal"
Super simple. Paint your metal as metal. But throw in some interesting colours, reds, purples, and yellows go a long way towards giving a sense of realism to metals. Also with stuff like aeroplanes and tanks they are usually made from a huge variety of metals, Chrome, lead, tin, aluminium, brass. Have fun and don't spend too much time on this stage. Just go mental on it.
Seal. Hairspray. Repeat.
You want the metal to be the very last layer that's seen when we get to the really fun part, so prepare it. Make sure its perfectly dry and blast it with a layer of GW purity seal. Wait for that to dry and give it a layer of hairspray. Don't skimp on the hairspray. The more expensive ones will dry quicker and wont leave a residue, which is exactly what we are after. I use L'Oreal (Because I'm worth it.)
(*Edit: The reason for using hairspray between the layers is to allow the paint to flake. Because it is a compound designed to dissolve in contact with water, by brushing water along it, it separates the paint neatly! It's an old technique but I haven't seen it done with multiple layers before, this will only work if you let the hairspray dry completely. I cant stress this enough!)
GW spray gun or airbrush, doesn't really matter which but a brush just wont work on this. (Maybe the primer cans might but I've never tried before. Might be too thick.)
The idea is to lay down a thin layer of paint to sit above the dry hairspray.
Layer 1 will be our damage paint. I've chosen a turquoise to contrast nicely against the yellow of the top coat and red of the rust.
Once the blue is completely dry, spray on another layer of hairspray. Don't worry if you can already see the paint cracking, that's just the settling process. until it gets wet it wont come off. Then gently do your final layer. Again, airbrush or spraygun. Just try not to blast it off.
Burn your butterflys.
This is my favourite part of the whole process. You've spent time on the base now you can see the fruits of your labour. Get an old toothbrush, wire wool, tissue, toothpick or whatever else you can use to scratch and destroy with. Dip the tool in a little water, (not too much) and gently scrape it over the surface in the direction you want to weather.
Now go too far and experiment. very worst case scenario is you scrape off all the paint and start again from stage 3! You can see which parts have been done with a little care and which parts have gone a little wrong. The nose I went mental on and I've ended up actually mixing the blue and yellow pigment, making a little green mess. the wings and tail fins however, I'm very happy with.
Rust, dust, and soot.
This is a fun step too.
I use a combination of actual rust.
(A tutorial of how to do this can be found here http://massivevoodoo.blogspot.co.uk/2010/01/tutorial-how-to-create-real-rust.html)
Modelmates rust effect.
Forgeworld weathering powders.
And various GW washes and inks.
You cant go wrong. You spill a shedload of black on it? Well slap some gloss over the top and It's tar or old crude oil. Do a streak of bright red? Well it turns out that rust does that too. It'll look good as long as you have a plan and prepare your base.
Finally pick out some edges with a brighter metallic colour, or not.
Hope this helps!