From the slight sheen to the paintwork,I'm guessing you've used Vallejo Robert? If so, I would definitely add a protective coat of Clear/Klear.
That's to avoid exposing bare plastic when you rub off the upper coat.
So, once the protective coat is cured, spray a coating of hairspray over the entire model. Allow to dry & then add the top coat. Apply the paint more thinly along the edges of panels which is where you'll want the top coat to rub off. Once that's touch dry, begin rubbing with an old toothbrush & plenty of water. Concentrate on the edges & raised parts to get the worn effect.
I have to add a note of caution ~ Vallejo paints are not known for behaving well when using the hairspray technique. Because of the plastisizer that's in them, the paint doesn't come off in tiny tiny bits. You're more likely to see it coming off in larger bits. Just something to watch out for.
If it was me... I'd spray the model with the top coat & then chip it with German Grey to show the worn areas. It's a more controllable method than using hairspray because you can add more if desired. With the hairspraymethod, once the paint has been removed there's no way to undo it.