I got a request from a reader this week for the copper roof tutorial I did a while back. I managed to dig up the photos and I've rewritten the text to go along with it.
It's real easy to do and like all weathering techniques, "less is more."
There are only three colors needed for this technique. Black, a suitable copper and a bright light-blue/green color. With the GW washes (this was done before them), you might be able to include those to add additional depth and enhance the effect.
I made up a quick section of roof out of foamcore and primed it black. Then I painted my base color of copper. Ultimately, we're trying to make the copper (roof in this case) look weathered like it's been exposed to the elements for a long time. You can apply the same technique for things like statues and armour on models.
When applying the patina (that's what the effect is called I believe) you want to add the blue in the recesses and areas where water might collect. If you were to do armour, it would be in the places that don't get much wear and handling.
I used an old brush to stipple some on along the ridges on my roof. Make sure to vary your brush movements so you don't end up with patterns in your paint.
With the blue in place, I went back over it with a black wash in some places to tone it down some and add additional weathering to it. Remember. it's not just this effect that will make your copper "weathered" but a combination of techniques that will get the best looking results.
Another trick is to take your metal color and drybrush it back over the blue making any raised edges look like they are "cleaner" and there is no buildup on those parts yet.