For painting the rusty metal on the models below I used the following tools/paints:
Citadel Leadbelcher paint
Citadel Runefang Steel paint
Citadel Agrax Earthshade wash
Forge World Light Rust pigment powder
Forge World Orange Rust pigment powder
MiG Pigment Fixer
Start by basecoating the metal areas on the model with Leadbelcher. Don't worry about highlights and shading at this point, but don't drybrush the metal either. You want a pretty smooth coat over the areas and just drybrushing over black won't produce a great outcome.
Next, you're going to put on TWO heavy washes of agrax earthshade. Be pretty liberal with the wash, but also take care not to have it pool heavily in some areas. The Ogres blade for example - if you put too much wash on the blade, it will run down and start going into the recessed areas on his hand. Be careful. Also, make sure that you allow ample time for the 1st coat to dry before applying the second coat. This will give the metal a brown, dirty look.
Let this dry. Usually I wait at least a couple hours for drying at this point so that the weathering powders and pigment fixers don't have any chance of mixing with wet paint. There are at least two pigment fixers out there on the market that I am aware of (MiG, shown here, and Secret Weapon Miniatures). I do believe they have the same effect and work the best, but isopropyl alcohol can be used to similar effect. If you've never worked with pigment powders before, there are several ways to apply, including dry, mixed, and as a wash. Each can produce a different effect and the techniques can be used together for a really nice finish. Here, we are going to focus on a little bit of washing and using some wet(ish) powders.
I just pour some of the powders into a small mixing cup and then use plastic pipettes to put some of the pigment fixer into each cup. Ideally, I'm looking to have some liquid pigment in some parts of the cup but leave some lumpy wet powders in other parts, as both will be applied.
I start with the Orange rust color, which is a bit darker and use both the liquid pigments to wash into the recesses of the metal and some of the clumps of wet powder to apply in some other areas. Use a lighter or heavier application depending on how rusty you want the model to look. Some of the pigments will dry very light and almost look oxidized, so don't apply large pools of the mix. I used a really old ratty Citadel Wash brush for this, so I wasn't able to be really precise - but this is supposed to be quick and easy, so it's not too important. Also, we're going to clean this technique up, so don't worry about edges and other mistakes - just try to keep it away from other parts of the model you have already finished.
After the orange rust, I repeat, with a little less application, with the light rust color. You want the rust to look patchy and textured, if possible. One of the best ways to see how rust looks and works, is to Google images for rusty metal. When you want to step up this process and make it look really nice, for Character or competition models, you can really go nuts with detail, even adding blacks and whites in for a very real finish.
These images show the model after the orange rust application:
And with the light rust on top of that:
Here the model is starting to look really nice, and you shouldn't have had to spend too much time applying these pigments. One final touch is necessary to make the model pop. If you've ever worked with real rusty metal, you'll know that as you drop pieces or bang it around, the rust that is removed from tension and blunt force will actually come off and leave shiny metal underneath.
To achieve this look, since the Ogre is undoubtedly smashing his sword, ironfist, and gutplate on everything he sets his eyes on, we are going to take runefang steel and highlight the edges of the model. Use a brush you would normally use for highlights and pick out rivets, edges, and any other area of the metal that is going to likely have it rough. On the gutplate for example, the protruding part holding the ogres pendulous parts is going to get a lot of wear, so you can apply the paint almost like a drybrush to get a great effect. See the pics below:
Thank you for reading. I hope this small tutorial helps you get the desired effect. It really is easy and you can add more time and practice into get really, really nice effects. Here are the finished models below along with some pics of models where I used more patience and detail to make this work even better. Thanks for reading, and happy painting!