[Arran]

To me it looks like there is more dirt then rust, a good cleaning will take care of most of it. The first thing I would do is take the dial off, sandwich it between two pieces of corrogated cardboard, and put it in a safe place. Then I would remove the tubes and try blowing off that filth with an air nozzle, outdoors of course. Some would differ with me over this but I would then give the chassis a was with some TSP or good soap to get rid of whatever dirt is left, since it will be a while before you power up the set the chassis will have lots of time to dry. With any luck there will only be a few spots to treat with some phosphoric acid or navel jelly and you can decide then whether it is worth dismantling the chassis any further to paint it..
Regards
Arran