Hi Alberto and welcome to the forum! Nice dio you posted in the other thread please post more of your great dioramas.
For this TINY truck I would suggest a totally different technique. This is the old Greenberg/Nash invention but it still holds up for this scale.
1. Airbrush a dark rust (enamel) color and let dry for two days.
2. Airbrush the original color of the truck. For a very rusty look a black coat with a few drops of white will work.
3. After a few minutes use a fine pointed brush dipped in thinner to reveal the rust in areas you want to show rust.
You can combine this old technique with the more contemporary ones and with a fine brush paint in chips afterwards. In my opinion this truck is too small for the H.S. technique and for the rest of the material you used. You can apply subtle streaks with powders after the first three steps.