For light rust dry-brush with enamel paint, I find Testors Rust the most realistic. Restraint is important here, a little bit goes a long way. For heavy rust, first paint the rusted area with a slurry of diluted white glue and abrasive cleanser (Comet or Ajax, etc.), Undercoat this area with dark brown and gradually drybrush the area with successively lighter shades of rust.
Rust streaks can be applied with pastels. However, only in extreme cases where an AFV has been left sitting in the rain for weeks will such streaks be evident - in other words, even though rust streaks may look very pretty indeed, do not apply them.
First of all, by grime I mean that black, greasy sludge that usually builds up on engines and things which get lubricated a lot. It is basically a mixture of oil and dirt. The best way to simulate this is with a heavy wash of black, with a hint of grey and green, paint. For extreme cases mix in a bit of dark brown chalk dust.
A convincing mud job is a little tricky to do; a bad application can ruin a model. However, by close inspection of references and a little practice, mud-slinging is one of the most satisfying aspects of weathering. I have concocted a good recipe for mud. The mixture consists of roughly equal proportions of the following:
- Tamiya paint (colour depends on what colour of mud is desired - usually I use Flat Earth).
- Tube water based paint (see previous brackets).
- Dry bulking agent such as Polyfilla, finely sifted earth or even coffee grinds, the desired consistency should determine which.
- Static Grass, (add an amount to suite your references).
- White glue - add only a few drops.
- Add water to the mix to reach the desired consistency, which should be about that of toothpaste.
A paste of baking soda and white glue can be painted on to simulate wet snow. For dry snow, use the same mixture but sprinkle on more baking soda. For thin, frozen snow, dry-brushing thick flat pure white paint works well; build up the layers and try not to get too much on the raised detail.