The Top Ten Tips of Texturing
One way to create dirt has already been covered, and that's photo overlays. Those are great for general wear and tear on your texture. If you want small specific details, you'll need to use other techniques. Below are a two techniques I frequently use, one for dust and dirt, and one for rust. Dust and dirt can be done very quickly with a solid brownish layer and a layer mask. Simply put the dirt layer at the top of your stack, and paint with a low opacity and flow. Rust is a bit more tricky. I used to handpaint rust, but it always had a bit of a cartoony look, and I was never able to get crispy rust that looks convincing and real. That was until DennisPls shared his technique with me, which I've been using ever since. Good damage placement only requires one thing: logical thinking. That can be chipped paint, scratches, rust, etc. There is a sure chance that you'll find dust and dirt in such an area. This doesn't only work for small borders.