If there is primer under the paint, and assuming that you don't want the paint shiny, I'd wet sand it with 1000# and in some places taking it barely down to the primer where you want it. Use discression here, small spots and in the right places. Now normally, fading works from the top down, so, look down from the roof and sand what you see. What you don't see...along the sides...shoot that with a light coat of satin clear. This should give you a pretty close approximation of a faded paint job without any shine. If you still want a little shine and not so flat, you might try a semi gloss clear...NOT satin. There's a difference, Im sure you know. The satin will make the paint look flat, like primer, the semi gloss with still leave it with a little almost shine, not flat.