On your car Can you sand an old faded paint job and just paint over it if the car body is in good shape?

Yes. If the paint is faded and oxidized, it might be worth a try cutting and polishing a patch and seeing how it comes up . The paint just under the surface may be sound. I have cut and buffed some sorry looking cars (red ones especially) and their owners have been stunned by the result. If you opt to paint it , wet sand it with 400-600 grit, but before you start, wash and dry the car , and use wax&grease remover twice over. This will prevent scrubbing contaminants into the underlying paint. Do it again before painting. Removing wax and silicone contamination prevents cratering or "fish eyes". Areas such as the roof should be well examined for sun damage such as crazing(a fine pattern of cracks ) such areas must be stripped back to the original primer, and then reprimed.