Modern scale model car kits come with detailed, multipart chassis that can be daunting for builders to paint and weather, but don’t let it deter you. First, start with the right base color.
In this case it’s the same as the body color, Tamiya Dark Blue (No. TS-55). But the color will depend on the subject — it might be gray or red oxide primer, E-coat, or just flat black. Paint lightly to retain the molded details; lacquers work great and hold up well to washes.

Pre-shade naturally darker areas by airbrushing Tamiya Smoke (No. LP-67) to create some shadows and enhance volume. Here, I painted the suspension components stock colors for a new car.

Give most of the components a wash of brown Tamiya Panel Line Accent Color (No. 87132) using the brush in the cap. Here is the top of the rear suspension arms immediately after applying the wash. It looks heavy, but when it dries, the opacity will diminish.

After painting it the appropriate colors, treat the underside of the engine and transmission with the same brown wash. Notice the rear suspension arms now that the wash has dried. The goal is to get a base of dirt and grime in all the hard-to-reach areas.

Apply the brown wash to the chassis’ recessed details. Let the solids settle in the bottle for 15 minutes. Then, with a medium, round brush, pull some of the liquid off the top for a secondary wash over most of the chassis — avoid the engine bay for a well-maintained vehicle.

After you paint the exhaust (click here for tips), mix Tamiya Flat Brown (No. XF-10) and Yellow (No. X-8) with Windex to make a rust-colored wash. Mix until you get the color you’re looking for. You can add a bit of baking soda to the wash to simulate a rusty texture.

After the rust wash sits for 5 minutes, flow brown Panel Line Accent Color along the outer edges of the rust patches. The enamel-based accent will not mix with the acrylics and leaves a realistic appearance.

Treat the muffler and resonator (if there is one) and let the rust dry for an hour. Go back with a paintbrush and rub off as much rust as you want to achieve the look you desire.

With all the chassis parts installed, the final round of weathering can be done — or you can stop right here. For my model, I used the rust wash from Step 6 on the rear brake drums to show surface oxidation.

Then I carefully misted, at low pressure, Tamiya Flat Earth (No. XF-23) heavily thinned with Tamiya Thinner (No. X-20A) — about 5:1 thinner to paint — over most of the chassis and inside tire walls. If it starts to run or pool, stop airbrushing and let it settle for a few minutes. Don’t go too heavy and stop painting before it looks too dirty; you can always add more later. The mist dries lighter and duller than when it goes on. If the weathering looks too heavy after a day of drying, you can wipe some of the acrylic finish away with a dry paper towel, or dampen it with water to remove more. A rinse of Windex will remove almost all of it, but be careful not to damage underlying acrylic paints.