This is the technique I used on a lot of my past terrain projects that I have posted here.
This picture shows the final effect we will achieve using this method.
I will list the materials I used here. You can substitute similar colours. The colours I used are listed in brackets.
-a sponge (I used a sponge brush and a makeup applicator, any sponge works. I often use blister pack foam. A stipple brush would also work.)
-dark brown (Americana Raw Umber)
-metal (boltgun metal circa 1997)
-reddish brown (Bestial Brown)
-light orange-brown (Vomit Brown)
-brown or orange wash (Gryphonne Sepia)
-orange ink (Tamiya Clear Orange)
I began by spray painting the whole piece using a matte grey primer. I then used my airbrush to add a little boltgun metal in the middle of the walkways to simulate wear.
1. Dark brown: Use your sponge to lightly stipple the dark brown colour in the areas where you want chips and rust. Dab the sponge into the paint and then try to get most of the paint off on some scrap paper. You want the sponge to create incomplete coverage. Dab the paint onto the surfaces using the sponge. I usually aim for about 20% coverage of a surface (change this up if you want more or less damage).
2. Metal: Use your metal colour to stipple metal inside the spots where you have already stippled the brown colour. Aim to keep most of the metal inside the brown areas to create the illusion of chips and damage. Some metal outside of the brown is fine. Be sure to leave lots of brown showing through. From this step on I used the makeup applicator as it has a smaller surface, giving more control.
3. Reddish brown: Now stipple the reddish brown (I used bestial brown) into areas where you want rust. I would recommend stippling the reddish brown over the exposed dark brown areas and lightly over some of the metal bits. You only want a little bit of reddish brown. Work with only a little paint on the sponge for this step. You can always add more later.
4. Light orange-brown: Using your light orange-brown colour (vomit brown), lightly stipple your rust highlight over the reddish brown areas. Like with the reddish brown, aim for just a light dusting in key areas. This colour really stands out. You don't need much. You are just trying to create the illusion of a little bit of corrosion and rust around the metal.
5. Brown wash: Now the fun part! Our rust and metal are mostly finished. To continue the illusion we want to create the effect of water and rust runs. Use your wash undiluted and draw vertical lines from areas with lots of corrosion. Create a variety of patterns. Large areas where washes pool, thin runs down the side, runs starting at rust, runs around rivets, etc. Try to ensure that the lines primarily go vertically, and don't slope to the side.
6. Orange ink (optional): This step is definitely optional. I really like to make the runs "pop" by highlighting them with some Tamiya Clear Orange. A second coat of Gryphonne Sepia also works. The use a detail brush and add a thin line of clear orange inside some of the rust runs. Don't overdo it. Just create a few gross and corroded areas. Then you are finished.
These are the steps I use. Obviously these steps can be modified for the colours you have and the effect you want to achieve. I have found this to be a quick and painless way to paint rust, corrosion, and exposed metal on larger terrain pieces.
Here are the four walkways I painted up while making this tutorial.
Here are some finished terrain pieces that used these basic steps to get the final product (click the link to see more).
Waste Water Reservoir
Industrial Terrain: Pencil holder
Hope this helps anyone who is interested in painting rust and corrosion and exposed metal on their terrain. This can also be scaled and done on miniatures. Enjoy.