The Salt method:
First step is to apply a couple coats of your favourite matt varnish to seal the model so that the water and salt does not remove the base/rust paint work you have already applied. Allow this to dry thoroughly. Lightly spray water over the model; I used my airbrush for this. Sprinkle the salt over the model in onto the areas that you want the rust to appear. I used Rock Salt and Table salt for this. The Rock Salt will give you a single large area of rust and the table salt gives you the effect of individual scratches also large areas of fading/rust.
I could have applied more salt to the bottom of the car but as this is a test I will do better next time.
As you can see I have a mixture of Rock & Table salt, again more salt would have been better, but I was able to correct this latter on whilst painting.
Allow the water and salt to dry, best to give it 24 hours. You can speed up the drying time by using a hairdryer set on cold and at a low speed. Also best to hold the dry as far away from the car as possible, you don’t want to blow the salt off.
Once it had dried I sprayed several thin coats of the body colour over the car. Again make sure you do not blow the salt off as the wet paint will make the salt come lose. I set the air compressor to between 5-10 psi. Once the salt was cover I turned up the pressure and applied additional coats.
The body colour isn’t that yellow in real life.