Here is a post of how i do all my rusted body panels on my car models. In the first picture i have posted you see the inside body panel of a 1970 Mustang and my Dremel tool with a round steel cutting bit mounted, you need to slowly grind the plastic down in the area of the dremel bit until its thin enough in some spots to see light through. In the second picture i have painted the ground down body panel gray to give some contrast for better viewing this is what it should look like after you are done. In the last picture you see the outside of the body panel and my scalpel knife pointing to the general area that you need to push the tip of the blade through the thinned plastic while you twist it this creates jagged holes and edges which gives you a scale appearance, i have also painted the outside of the panel a rust color to give you an idea of how it will look, not bad for about 15 minutes of work.
