Shuttle - looking dirty

Gidday!

I know that there are a lot more important problems to worry about with the
return to flight but after trolling through various images on the Nasa site,
one thing that stands out, in particular with the night shots of Discovery
on the launch pad, is how dirty or marked Discovery appears.

During the time that its taken to prepare for the return to flight, wouldn't
Nasa have wanted to paint the shuttle, if not for protection, just for
public relations? If you think of Discovery as the Space equivalent of Air
Force 1, then comparatively every shot you ever see of Air Force 1 shows it
in pristine condition. Are all the Orbiters like Discovery?

As I say, I know there are more important issues but this is just something
I've noticed and thought that people who know much more in this area than I
do (I'm just an enthusiast from Oz) could shed some light on.

Regards,

David.