After reading on some forums that the rear barn door aperture is quite susceptible to rust, I decided to check mine. It was just starting to go but I managed to catch it before the tinworm took too bad a hold.
A quick sand back with a wire wheel, cure-rust treatment and repaint with some black hammerite (its hidden behind the door seal) sorted this bit out above the doors.
There was also some rust coming through where the door seal had worn away the paint, I reated this, primed it and sprayed with some gloss black. Its not a perfect match for the Kuro black paint of the Troll, but being pretty hidden its not noticable.
While I was tackling this, I noticed a few small patches of surface rust on the roof of the truck as well. Just a few random spots where it seems the paint had been broken by flying road debris. Again, a quick sand and paint sorted these out.