The following technique was discovered by mistake while attempting to lift the paint from a tiger with the use of an oven cleaner...
I found out that aluminium objects were heavilly coroded by the cleaner called "Fornet".I think it's exactly the same as "Mr Muscle".
Since an image is a 1000 words let me show you the little experiment i performed today.
After shapping the aluminium(coffee can sealer) as desired,i placed it into a sealed bag with the cleaner.


The result was the following amazing texture

As you can see it "ate" the metal all the way through!!

After that,the texture on the material itself is guiding your steps!
Glued and primed with red oxide-


Left it to dry completely~10h
And started painting it.
First a very thin acrylic wash with a mixture of rust matt+leather matt and in some areas with brown matt added to the above.
Preselected areas are then painted with an enamel mix of ~60%flat black+20&gun metal to give a little shine+20% of german grey to make it a little lighter.
Final touch,with CMK pigments "dark rust" and "Vietnam earth", and the result is this.
Putting the pigments on-

Brushing the excess off-


NOTE
After you take it out of the bag brush it with a toothbrush to clean it from the white pouder that is formed on the surface.
WARNING
Put your mask on!!!!!!!!!!!!!!
And there you have it!!!
Fast,easy,simple!!
An other bonus with this technique is that you control the ammount of corrosion you want to accieve by checking it every few hours untill you like what you see and since the cleaner is a foam(not liquid like vinegar) you also control the exact place you want to rust!!
I haven't tryied it yet on another metal like copper but will do and let you know of the outcome!
I hope you like it and find it usefull!!
Tasos