I was looking through some of my old 40K photos and found some step by step images from an old tutorial I did on making a rust/weathered paint effect for vehicles.
Start off by painting the colour you want to be showing through under the paint. Most commonly this would be bare metal or a rusty red colour which is what I’ve gone for here. I can’t remember which colours I used exactly but a mix of red and brown is commonly used.
What you need to do next requires too items you wouldn’t usually use when painting. These are salt and hair spray. Sprinkle salt randomly on to the model and then use the hair spray to hold it on.
Once this is dry you then paint the base coat for the model.
Once your base coat is dry you can then start to remove the salt from the model. This will then leave the rust colour showing through underneath. You may be required to use an old toothbrush and warm water to do this depending how easy the salt will come off.