You will need:
Krylon Dark Brown Flat Camo spray
Two large flat wide bristle brushes (drybrush)
Citadel Bestial Brown
Citadel Tau Light Ochre
Black wash (if necessary)
You don't have to use Citadel colors. Use whatever mid tone generic brown you like, but it helps if it has a reddish tint to it. I find Bestial is the best color for this. If you are using the new paints, you might have to find out what color replaced Bestial Brown. I use Tau Light Ochre, which used to be called Vomit Brown. Again, use whatever color that you feel is a good replacement. The key here is that this color has a bright orange tint to really make the rust pop.
|Not pictured: Citadel Ironbreaker (or chainmail).|
Next, grab a large flat bristle drybrush and begin applying the Bestial Brown. You do not want to drybrush this layer. Actually, the opposite, You should be stippling the color on in the recesses (drybrushing brings out the highlights-rust appears in areas where water sits and along edges between). You want to put the paint on the brush, and then just like you would be drybrushing, remove most of it first on a paper towel to get most of the paint out. Then, stipple from the edges of the piece inward and on and along seems and joins or areas where rust would appear. Feather the color out as you go to blend it into the base color.
|First stippled coat of Bestial Brown.|
Next, you will do the same thing, but adding Tau Light Ochre. It is important here to NOT clean your brush. You want to dip your brush into the Ochre and then brush it out on a paper towel and get the previous color to blend on the brush. Begin stippling again, but this time you want to start hitting the edges and lightly drybrushing and stippling at the same time to pull the colors together while not hiding the base colors or the first layer.
|First coat of Tau Light Ochre and Bestial Brown mixed on the brush.|
Next, get a clean brush and do the same drybrush stippling technique on all of the edges and areas where you want the rust to be brightest or concentrated. This color will really pop, so it is important to apply lightly and feather out or there will be too much contrast. I like to stipple this color in areas where water would pool and rust would be the most concentrated. It's also good for the sharp edges and places where you want joins or seems to pop.
|Final highlight of Tau Light Ochre.|
Finally, you should add a very very light drybrush of a bright metallic such as chainmail. Here you just want the very thinnest edges and places where metal would wear to appear. This pulls the whole thing together and makes the detail pop out. It is easy to go overboard with this, so for me, less is more.
Here is the roof section weathered and in place in the unpainted but base coated building. There's lots you can do with this if you weather according to how rust actually forms and where water would pool or lay. It doesn't have to be entirely realistic however. You want to give it more of the overall feel of rusted and worn metal.
I have found that if you get too heavy with the drybrushing and stippling and you get too light with the colors overall (where the base color starts to disappear), then you can wash the piece with an overall black wash (or just in areas you want to darken) and then touch them up again. Or, if you want a darker and grimier overall look, a final black wash can pull the colors together and darken it all back down. I have found I have only needed the wash on the flatter pieces. The spots with lots of texture and detail generally don't need it. But, it's your terrain, so apply as you like to suit your specific needs.
For rusty water drips and pools I like to mix a wash of Tau Light Ochre with a black wash and then "drip" it down walls and in pools where rusty water collects or stains.
Below are some shots of different pieces from the project that have been weathered this way.
|Rusty water stain under the door.|
|Rusty water stain along the edge of the floor and on ladder.|
|Rusted corrugated sheet.|
|Rusty water drips on wall.|
|Rusty steel mesh grill.|