I recently posted my own solution to creating "rusted" iron on the original posting in the "diorama" forum, but I'll also put it here.
This method should work very well for simulating the look of built up corrosion on either iron or copper items.
First, grind up some perlite (available in most well stocked garden supply shops) to the consistency of baking soda.
Second, mix your paint to your prefered tone of reddish brown for iron oxide or greenish blue for copper oxide. Apply a smooth base layer of this paint to your subject piece.
Pour some of the remaining paint into a saucer or other flat container and gradualy mix in the crushed perlite. Continue mixing until the perlite has become completely covered in the paint, you don't want any of its original white color showing through. After mixing, thin the mix out as you would if you were preparing paint for an airbrush.
Next, take an old toothbrush and dip its bristles into the paint, making certain that some perlite is picked up. Point the bristles downward and pull your finger across them, toward you, so that the paint is thrown onto your subject. You'll want to spray excess paint onto a paper towel or something first before aiming it at your model.
The perlite will give your finished paint job the same gritty appearance that iron or copper oxide has. Also, it will allow you to build up the paint in spots, so that you can achieve the lumpy, uneven texture that long standing buildups of this sort of corrosion often have.
Good Luck!